Point of no return
In many Mario games, particularly RPGs, there is an event or cutscene in which during or afterward, the player becomes incapable of saving their progress. This event is often referred to as the point of no return due to the player being unable to return to any previously visited locations to complete additional sidequests or collect additional items upon reaching a certain point in the game.
The point of no return usually comes in the form of a cutscene, during or after which the final boss is fought. If the player loses this battle, they are able to continue from their last save point.
Typically, a character or message in the game informs the player of the upcoming point of no return, giving them one last chance to save their progress and make additional progress. Occassionally, said character will even give a motivational or encouraging "pep talk" before the player is given the option.
Mario & Luigi series
Once Mario and Luigi reach Bowletta's throne in Bowser's Castle, she gives them a speech in which she reveals her plans to take over the Mushroom Kingdom as well as the Beanbean Kingdom after defeating the brothers. After the brothers defeat her, once and for all, Prince Peasley appears to inform them he has placed an explosive in the castle set to go off within a few short minutes. At this point, Mario and Luigi can only take the shortcut to escape the castle in time with Blablanadon. After this is accomplished, the final cutsence plays, with the credits following shortly thereafter. If Mario and Luigi do not escape the castle in time, they must start from their last save point, before the battle with Bowletta.
In Mario & Luigi: Partners in Time, the point of no return comes with quite a few exceptions.
The initial point of no return comes when Mario, Luigi, Baby Mario, and Baby Luigi enter the room in Shroob Castle in which Princess Peach is being held captive. Before they are able to open the doors to this area, Stuffwell asks them to save their progress, informing them that they likely won't be able to return. After convincing them, Mario, Luigi, Baby Mario, and Baby Luigi are able to save their progress, then meet up with Peach.
After a cutscene and a minigame in which the Shroob Mother Ship is taken down, Princess Peach is rescued, the Princess Shroob sisters are defeated, and everyone returns to their own time. It is here that Shrowser is revealed to be the true final boss. Once he is truly defeated, everyone truly returns to their own time.
However, even after Stuffwell's lecture, the player is given three opportunities to warp back to just before the point of no return, via hitting the Save Albums before taking down the Mother Ship, after taking down the Mother Ship, and after returning to Peach's Castle, just before entering the throne room, where Shrowser is fought. Due to this, the true point of no return is when Mario, Luigi, and the others enter the throne room.
Mario & Luigi: Bowser's Inside Story / Mario & Luigi: Bowser's Inside Story + Bowser Jr.'s Journey
After Bowser defeats Super Peach's Castle of Fury and Dark Bowser flees, Starlow warns Mario and Luigi that they should complete anything else they wish to do soon before it can no longer be completed.
The true point of no return in this instance comes when Bowser confronts Dark Bowser atop Peach's Castle. He summons clouds of darkness that spread across the Mushroom Kingdom and overcome the inhabits, such as Toads, a Goomba, a Koopa Paratroopa, and Broque Monsieur. In the midst of this, Starlow tries to reveal her true identity to Bowser, but cannot be heard for the commotion. Instead, she reminds Bowser he can count on them during the final fight.
After Dark Bowser and the Dark Star are defeated for good and peace is restored throughout the kingdom, Bowser attempts to kidnap Peach as Fawful appears in his body to speak to Mario, Luigi, and Starlow. Fawful gives his final speech before giving up and exploding, launching Mario, Luigi, Starlow, Toadsworth, Toadbert, and everyone else out of him. Bowser finally realizes it was Starlow giving him advice and helping him out the entire time. Furious, he proceeds to fight Mario and Luigi but is defeated himself.