When a student tells you that you didn't explain or teach something to them, but you clearly remember that they were in class when you taught it.
OK students can have short memories, but the instance the other day probably hurt more because the student seemed to give off an impression that not only did I not do it, but I was trying to gaslight them.
Given that at least one third of the class seemed to be getting on with it in some capacity, this sort of annoyed me as it wasn't that I had genuinely not taught this thing, or had done such a bad job at it that it was just like I never taught it.
While students may not be expected to learn entirely by themselves without any guidance (even the really good students), at some point good students (not just in grades, but in behaviour/attitude/habits) must exercise a degree of initiative.
OK rant over... teacher out...