This problem is rampant in lectures and academic talks. Many times, a speaker will be saying something which to her is of course obvious, and she'll put it up on the board and ask if everyone gets it. There'll be a lot of blank faces and maybe one guy nodding reflexively. And she'll be like: "OK, if everyone gets it we'll move on"; when actually no one would've got it.

Many times the lecture is just a duty amongst pursuits more interesting, and the speaker secretly hopes that nobody asks, and it shows.