The “Who Knew?” File of Usage Errors or Usage Controversies

I’m collecting examples of usage errors or usage controversies that bite people unexpectedly. In particular, what I have in mind is words or phrases that meet both of these criteria:

(1) You once — preferably after the end of your college education — confidently thought that the term, or a particular way of using it, was uncontroversially correct and effective.

(2) You later learned that many people think the term or the way of using it is wrong, or that the term also has a different meaning that makes it less effective at conveying the meaning you thought it conveyed.

An example for many people might be “fulsome.” As best I can tell, many people confidently think it simply means “thorough” or “comprehensive.” They are not aware that some people think that it only means excessively effusive to the point of being offensive. Nor are they even aware that one of the meanings of the word is excessively effusive. They therefore use “fulsome” to mean “thorough,” without realizing that such a use is likely ineffective (since it alienates some readers or listeners, and counterproductively brings up a negative connotation in the minds of others).

I know this sort of thing has happened to me (though not with the word “fulsome”), but I forget exactly with what. So I’d love to hear some examples from our readers’ lives.