Word meaning the feeling of thinking there are more steps on a flight of stairs than there are

I remember reading about a word describing the weird sensation felt when one reaches the top of a flight of stairs one step sooner than one expected to.

The phenomenon in general is known as a phantom stair or phantom step. The sensation is one of stepping through a phantom step.

Urban Dictionary has an entry on phantom step:

When you walk up the steps, and feel like you haven't quite reached the top yet, you will always hoist your leg up a bit higher just to make sure you won't tumble. But low and behold, you had already reached the top. The phantom step is always followed by a feeling of confusion and a sense of disappointment.

Vladimir Nabokov, one of the greatest prose writers ever, uses phantasm of a step:

Another part of the ritual was to ascend with closed eyes. “Step, step, step,” came my mother’s voice as she led me up—and sure enough, the surface of the next tread would receive the blind child’s confident foot; all one had to do was lift it a little higher than usual, so as to avoid stubbing one’s toe against the riser. This slow, somewhat somnambulistic ascension in self-engendered darkness held obvious delights. The keenest of them was not knowing when the last step would come. At the top of the stairs, one’s foot would be automatically lifted to the deceptive call of “Step,” and then, with a momentary sense of exquisite panic, with a wild contraction of muscles, would sink into the phantasm of a step, padded, as it were, with the infinitely elastic stuff of its own nonexistence.

Nabokov, Speak Memory (1966)

Whether or not this passage is the origin of the above expressions is an interesting (and open) question. It seems likely, however, that phantom stair and phantom step are formed on analogy with phantom limb, rather than derived from the Nabokov passage.

Probably something to do with your body's proprioception, that is the sense of where your body's parts are and helps you anticipate where to place your feet when walking. When you get used to walking up or down a set of stairs, your mind has an expectation of where the next one will be which is why you don't have to test the distance of where to put your foot on each step. Your brain is already telling the body where the next step will be. This is also why building codes have such strict rules on stairs. If you have a riser that is off an inch or two, it often causes individuals to stumble and fall down the stairs.