Flipper

One-Way Flippers are doors that, once passed through, prevents access to the area before. Players who find a set of these in a level should remember to make sure they have all the desired items/collectibles before passing through the flippers.

They are commonly referred to as "pinball flippers" due to their resemblance to flippers from a pinball game.

Originally debuting in Super Mario World 2: Yoshi's Island, the flippers showed up again in Super Mario Advance 4: Super Mario Bros. 3 during the stages Slidin' the Slopes, Doors o' Plenty, and Swinging Bars of Doom.

A different type of One-Way Flippers appears in New Super Mario Bros. in World 2-3 only. These types are wooden flippers with red arrows on them indicating which way it opens to, and are found individually rather than in pairs. These types of One-Way Flippers were originally going to appear in New Super Mario Bros. Wii; however, they reappear in New Super Mario Bros. 2 in a few levels.