Pet peeve: discovering that a word I've been using doesn't mean what I thought it meant. It makes me unreasonably sad. Especially when I can't think of any other words that mean exactly the same thing.

First "touchstone," now "diffident." Dammit. :C

Edit to add: I have at least figured out that my wrong reading of "diffident" is probably the result of conflating it with "indifferent," but that still doesn't quite mean what I was using "diffident" to mean.