Amount of people
This is one that a good portion of the population is guilty of. I hear plenty of people use “amount” while referring to discrete objects, such as cars or people. (Yes, I just called people objects.)
I don’t remember actually learning this rule, but I have always used “amount” while referring to things that do not easily separate into countable parts, such as water, sand, courage, experience, etc. It seems to me that “number of people” (or some other phrase, depending on context) should be used instead.
I understand that there are cases where this can get confusing (”amount of time” but “number of minutes”), but I think it’s never okay to use “amount” with something that is thought of as a collection of separate objects. Am I crazy? Does this make anyone else cringe? I don’t think I made this rule up, but I will concede that it’s a possibility.