If you are jumping and move on to a platform horizontally - often when pushing up against it - there's a chance that your player won't land but instead get stuck in a sort of jumping/landing limbo. The only ways to get out of it are to jump again, walk off the platform, or stop moving horizontally in which case the player will do a short "hop" that is actually the remainder of the previous jump.
Here is a .capx to easily reproduce this bug.
The player is yellow when on the floor and orange when not. Just hold right and jump to that higher platform while moving and chances are you'll "land" but remain orange.