Missable content refers to items, equipment or other details that can be gotten at one point in the game, but can be missed permanently if the player does not obtain it before reaching a certain point in the game, or before its location cannot be revisited.
In Mario role-playing games, rare items and accessories are often more difficult to obtain past a certain point, and in some cases, missable content is rare to begin with (such as being rare drops from bosses), but not all rare items are missable.
Super Mario series
In earlier copies of the game, only one of the three Challenge-related pictures in the Album (for collecting all Red Coins, collecting all Yoshi Eggs, and reaching all target scores) can be obtained.
If the player loses eight lives in a course from one of the first eight worlds before completing it, a Super Guide Block appears when the player re-enters that course. If the player saves the game after seeing one, it permanently prevents the player from ever having shiny stars on their save file.
Similar to New Super Mario Bros. Wii, if the player loses five lives when attempting a course from one of the first eight worlds, an Assist Block will appear on re-entry, permanently preventing the player from ever having shiny stars on their save file.
In all subsequent Mario platformers, making the Super Guide Block or Assist Block appear will not void the player's chance to get shiny stars on their save file; any player who uses the Super Guide or Invincibility Leaf can regain shiny stars by completing the same level(s) without using it.
There are three Checkpoint Flags that completely disappear once the player has reached a certain point (regardless if they have been activated or not):
In the Luncheon Kingdom, there is a large Coin Stack on the left of Peronza Plaza behind some giant vegetables. After obtaining the kingdom's first Multi Moon, this Coin Stack disappears, even if it was not collected.
From October 27, 2018 until November 7, 2018, for the first anniversary of Super Mario Odyssey, 39 pixel Luigis and Captain Toads were added in the ground around the concert of New Donk City in the mission A Traditional Festival! Mario could explore the area and ground pound their spots to reveal them, each pixel character awarding him 200 Coins at first discovery.
The player has only one attempt to collect the Frog Coin inside a Surprise Box when Mario and Toad enter the Mushroom Castle's foyer for the first time, by jumping on Toad's head and climbing up to the Surprise Box at the end of the room. If the Surprise box was not obtained, leaving through the entrance before entering the throne room and then returning will reset Toad's position and allow the player to try again if they fail to reach the alcove.
The player has two opportunities to collect the items in the vault of the Mushroom Castle: 10 coins, a flower, and a mushroom. The two times these can be collected are right after meeting the Chancellor, and during the Shyster invasion. Even if these items were collected the first time, they reappear in the second time. Once Mack has been defeated, the vault is sealed, preventing these items from being obtained.
Paper Mario series
During Princess Peach's segments, there are a few chances to use the Mysterious Treasure Chest to deposit items and Badges, the last opportunity being the segment just after Chapter 6. Once Peach's segments are done, all items and badges she has collected (but not deposited into the chest) become unobtainable.
The player can miss out on an opportunity to fight the Anti Guys Unit in Bowser's Castle should the player correctly answer three questions from the second Bowser Door. The fight however does not contain any exclusive items and its purpose is for earning extra Star Points.
Upon opening the chest containing a swarm of two hundred Boos in the basement floor of the Creepy Steeple, the player can be rewarded with an Ultra Shroom if they answer "200" to the Boo asking how many Boos were released. Otherwise, they are rewarded a single Coin.
Extra Flower Finder badges can be obtained as a rare drop from an X-Naut PhD. However, once Mario and his partner teleport back to Rogueport Sewers at the end of Chapter 7, they will not encounter any more X-Naut PhDs. This makes extra Flower Finder badges impossible to get after this point, even once the player has regained the ability to return to the moon in the postgame.
When the player faces and defeats Gloomtail in the Palace of Shadow, the player has an opportunity to blow up the eastern wall of the chamber with Bobbery to reveal a passageway to a hidden room with two ? Blocks containing an Ultra Shroom and a Jammin' Jelly. If the player doesn't obtain the items after setting the Star Key in the pedestal in the Riddle Tower, they cannot return to Gloomtail's chamber again to collect them.
In Chapter 7-4, there is an area where a Nimbi locked himself in the chest. Before the chapter is completed, the player can open the chest and receive an Ultra Shroom Shake in return. Once the Chapter is completed, the player will be unable to open the chest if they come to the place again.
After subduing one of Wiggler's segments during the storyline of World 3, the player has one chance to bring each one to a specific location to create an entry for the optional Wiggler's Diary before being brought to Wiggler's Tree House. Once Wiggler's body is reunited, the player will not be able to obtain any more diary entries they may have skipped.
At a Dress-Up Photo Studio in Shogun Studios, Mario can take pictures in a photo booth with Olivia and Bob-omb wearing different outfits. This is possible only if Mario returns to Shogun Studios immediately after arriving at Sweetpaper Valley. After Olivia gets crushed by a boulder, any pictures taken from that point will display only Mario and Bob-omb. After Bob-omb's sacrifice, any pictures taken from that point will no longer display him with different outfits and instead have him as a spirit with his standard appearance.
Prior to version 1.0.1, it was possible to miss a Toad turned into a surfboard in Shroom City should Mario leave the area after shutting off the water without rescuing the Toad, which in turn would also prevent Mario from getting a +10 HP MAX UP Heart that the Toad rewards him with.
Inside the foyer of Bowser's Castle, where Koopa Troop members are fighting the Folded Soldiers, Mario can optionally fight the Folded Soldiers and earn 1,000-coin sacks from the minions only after defeating the Folded Soldiers in actual battles without placing any unfolded minions in his range of attack. However, this is only possible before Olivia is snatched by Handaconda, as the Scissors cuts up the minions later on.
In Hotfoot Crater, there are two coins and a 1,000-coin sack on a platform south of the airship remains, and a breakable crate sitting on the remains containing another 1,000-coin sack inside. If the player exits the remains to the eastern platform without collecting any of them, they will become inaccessible, as the Paper Macho Goombas block up the airship remains.
In the final stretch of Origami Castle, Mario can hammer a wobbling King Olly statue and cause it to drop onto a flippable bridge to instantly defeat the last hoard of Folded Soldiers. If he doesn't do this early, he goes straight into battling them.
In this game, the player can get special achievements by completing the game without using any battle accessories or getting a game over, which adds banners to the ending image and more icons to the file select screen (a heart for no game overs, and a Mario glove for no battle accessories). However, if they get a game over or equip any battle accessories before beating the final boss, these achievements are lost forever, since Mario no longer meets the requirements for them. Accessories that only have effects in the overworld do not affect this.
Luigi's Mansion series
The player has only one chance to capture Speedy Spirits in dark rooms: if the player fails to catch one, it will not appear again, even during the blackout. The player however has enough opportunities to capture Gold Mice as long as a room is dark: should the player fail to catch one, they only need to exit and re-enter the room, and if there is a piece of cheese, to re-scan it with the Game Boy Horror.
When sucking up Portrait Ghosts, the player must try to capture them in one attempt to collect all the pearls they drop. If the suction breaks in the middle of an attempt, the player may not be able to collect medium-sized pearls or a large-sized pearl.
In the Boneyard is a lone plant in the garden that can be watered. If the player does so before fighting Bogmire, after defeating him, and after defeating Boolossus, it will grow and reward the player with ample amounts of treasure. If the player fails to water it after defeating either boss, it will wither and die.
In the Boilerworks, there are some coins that require Gooigi to open up a circuit box on a high ledge to collect. This is only possible while the water level is raised, since a chain drops down and blocks the path later on.
Later in the game, Luigi revisits the Boilerworks to rescue a Toad that was sent by E. Gadd to retrieve a part for the Poltergust G-00. In this part of the game, Toad can be fired at walls and objects to knock them down and reveal money, as well as open a shortcut back to the start of the area. If Luigi doesn't do this stuff at this point, he'll be unable to do so later, since no alternative projectiles are provided for knocking down most of these walls and objects on repeat visits.
Mario & Luigi series
The player only has one chance to collect all the items from ? Blocks in Peach's Castle, both visits of the Koopa Cruiser, Chucklehuck Woods Underground (the cavern area just after defeating Chuckolator), Guffawha Ruins, and the S.S. Chuckola, as these areas become inaccessible after leaving them.
The player has until the second destruction of Beanbean Castle Town to collect all ten Beanstones scattered about the western and eastern halves of the town to obtain the Bros. Rock accessory, as well as collecting all five Beanlets to earn a Golden Mushroom. The southeastern house on the western side contains a regular ? Block and three Hidden Blocks containing Hoo Beans, and the northern doubled-entrance house on the eastern side contains four ? Blocks. After the second invasion, the player will be unable to unearth Beanstones and locate the Beanlets, and the houses containing the ? Blocks will be locked for the remainder of the game.
Several bosses and enemies drop rare equipment that can only be gotten by equipping the Game Boy Horror SP simply by defeating the boss or enemy, or by using the Advanced Swing Bros. to steal it from them without equipping the former:
The remake Mario & Luigi: Superstar Saga + Bowser's Minions retains the missable content from the original version. While some item and badge drops have changed, bosses that carry any items only have one and are guaranteed to drop it. Nevertheless, the player can still miss their chance to obtain them from several bosses if they flee from battle when they're the last one remaining:
In Guffawha Ruins, the Mari-Lui Badge can be found at the dead-end east of the fourth area, in the ? Block on the right. Elite Chuck Guys and Limbo Bros now carry items that can only be obtained if Luigi defeats them while equipped with the Game Boy Horror SP: the Elite Chuck Guy Badge from Elite Chuck Guys, and the Defense Pants from Limbo Bros. Since the amount of beans required to make all seven blends from Starbeans Cafe in this game has been halved from the original version, the player can obtain the Game Boy Horror SP much earlier and have Luigi equipped with it at that point. Because the gates to the Guffawha Ruins are sealed after Bowletta kidnaps Peach once, the player has only one chance to obtain these three items.
Several bosses have a 15% chance of dropping rare equipment, even though there are duplicates of said items found elsewhere.
Mario & Luigi: Bowser's Inside Story / Mario & Luigi: Bowser's Inside Story + Bowser Jr.'s Journey
The player only has one chance to collect all the items from ? Blocks in Peach's Castle at the beginning of the game.
The player can miss out on an opportunity to do quizzes with the three Nutsoglobins in the Flab Zone if Mario and Luigi answer the questions incorrectly. The quizzes are entirely optional and its purpose is for earning coins.
Like the previous games, several bosses and "support enemies" can drop rare equipment, even if there are duplicates of said items found elsewhere, or dropped by enemies with lower drop rates.
Much like the first remake, Mario & Luigi: Bowser's Inside Story + Bowser Jr.'s Journey gives all items dropped by bosses 100% drop rates. Additionally, there are no bosses that flee from battle, so all boss drops are guaranteed to be obtained, including the Shroob Boots, meaning that if the player decides to fight the Shroobs, they will get three pairs. Nevertheless, the player can still miss their chance to obtain extra experience points from several support enemies of bosses if they flee from battle when they're the last one remaining:
While almost all bosses in the game always drop a rare piece of equipment upon defeat unlike the previous three games, Mammoshka is the only one to drop a potentially missable accessory, the 1-Up Gloves, with a 50% chance of being dropped.
All missable items in this game are beans from drill spots and items from ? Blocks in Bowser's Castle, as this area cannot be revisited. These items are the Spin Hammer, the Bros. Wear, and the SPEED Mush Jams.