Word for instantaneous mismatch of expectation and reality

Your question is asking for a word to describe a feeling.

Candidate terms include:

• confusion or surprise (the most general terms)

• abashment (the most precise term, in my opinion) - [A feeling resulting from consciousness of error or presumption, etc.]

Bemusement: the state of being bewildered; puzzlement

Befuddlement: The state of being confused

Consternation: the state of being helpless or unrelieved due to the happening of something unexpected; anxiousness

Unforeseenness: the quality of being unanticipated or unpredictable

Abyssalness: unfathomableness; inscrutableness; irresolvableness

Nadir: the most unsuccessful or lowest point in a situation

Hinderedness:

The quality of being obstructed

incapacitation:

The fact of someone being deprived of his/her ability to do something

I hope this is helpful. :)

Sounds like the persons in question have misjudged the situations or made misjudgments. From Lexico:

misjudge: Form a wrong opinion or conclusion about; make an incorrect estimation or assessment of.

The feeling such a person might experience upon discovering their misjudgment? Good possibilities includesurprise or incredulity, as suggested by @JasonBassford in a comment above. From Lexico:

surprise: A feeling of mild astonishment or shock caused by something unexpected.

incredulity: The state of being unwilling or unable to believe something.

The staircase situation specifically is phantom step.

The mismatch between expectation and reality might be something like cognitive dissonance but I'm not entirely sure. I don't think there is a specific word for the feeling you want to describe.

However, I think any of the following words/phrases would work well:

Caught off balance: To startle one; to come upon one unawares or unprepared. Usually used in the past tense.

Example: The security guard caught the thieves off balance. [TFD]

Startled: If something sudden and unexpected startles you, it surprises and frightens you slightly. [Collins Dictionary]

You could say you were startled.

You could also use caught unawares, dumbfounded or bewildered.

Jarring and incongruous could also work.

False summit: In mountaineering, a false peak or false summit is a peak that appears to be the pinnacle of the mountain but upon reaching, it turns out the summit is higher. False peaks can have significant effects on climbers' psychological states by inducing feelings of dashed hopes or even failure. The term false peak can also be applied to non-mountaineering activities where obstacles posing as the end goal produce the same psychological effects. [Wikipedia]

The fairly formal verb confound is in this area:

confound 1.

cause surprise or confusion in (someone), especially by not according with their expectations.

the inflation figure confounded economic analysts

[Lexico]

What many would label the gerund from this verb has been used, but this usage is formal and old-fashioned:

... [M]uch to my confounding, many of my sensitive colleagues on this side of the Chamber, whom the very word 'subsidies' causes to go into political hysterics ...

[US Congress; Senate Committee on Commerce_Steamship communication to South America, 1910]

Nonplus is a verb with a similar meaning, and the noun nonplusment appears in Wiktionary but is marked as rare..

There is the phrase rude awakening:

: surprising and unpleasant discovery that one is mistaken
// He thinks he can get by without doing any work, but he is in for a rude awakening.
_{Merriam-Webster}

Discombobulation springs to mind.

Discombobulate
to confuse or disconcert; upset; frustrate: