Without such drastic admissions, are you really asking whether someone's opinion of you can radically decrease without you doing anything that seems out of the ordinary to you?

Essentially, yes. The question wasn't particularly clear, I admit, because I don't know how to phrase it more clearly, except for individual examples, and I want a slightly more general answer.

But, to give one example scenario of the kind I have in mind, take this: we know that appearing confident is important for the first impression. What happens if a person has formed an impression of you as confident, but later you display some clearly non-confident behavior?

There is the fundamental attribution error working against you, but there's also the fact that people in general don't like updating. And as I said, my personal experience shows such high variance that I feel very clueless; I've seen decent people who remain friends with others I would long have thrown out of my social circle, and other people who irredeemably condemn you as soon as you commit the slightest blunder. Somehow the latter group seemed to be more pathological than the first, for independent reasons, but still… I feel a desperate need for data.

I guess, but you also have to keep in mind the difference between what the other person thinks and what s/he is willing to show.

I'm keeping that in mind all the time, which is why I called the signals noisy. They are, to an annoying extreme.