The phenomenon of (for no particular reason) subconsciously mis-counting the number of stairs in a flight of stairs, and (nearly) falling.
It seems to happen more often in the dark, and particularly when one is tired or distracted. It has variations too -- forgetting the last stair, and thereby missing the floor when it is lower than expected a step later, or allowing for one extra stair, and stumbling when the floor is higher than expected.
If people are present, it has the side-effect of turning one's face red, due to the embarrassment of seeming uncoordinated. If people are not around, one normally stumbles, recovers, then promptly forgets about the whole thing. | 
