Disappearing platforms

This is applicable for both actual floor that disappears, crumbles, folds out or whatever as well as inexplicable "platforms" that do the same.

These may disappear when the player steps on them or they may repeatedly disappear and re-appear regardless of player interacting with them. Such as a platform that folds to the wall and then folds back out, and repeats that motion. Alternatively it could be a flexible surface that can't support the player character's weight (and thus bends down with it) but rebounds back to original position once any weight on it has been removed.