Flipper
One-Way Flippers are doors that, once through, one cannot get back to the area before. This can prove to very annoying if the player exits through the doors and realize they forgot to get something important, such as a check point or a power-up. It forces the player to search everything that the area can offer rather than just running through the level as fast as possible.
They are commonly referred to as "Pinball flippers" because their shape resembles two flippers from a pinball game; just like in the original pinball, once an object passes through, it cannot and will not come back.
Some levels where these doors show up are Slidin' the Slopes, Doors o' Plenty and Swinging Bars of Doom. A few also show up in Super Mario World 2: Yoshi's Island.
