Quite often! Sometimes words just fail me. I can't remember what I meant to say or I stumble all over, "tongue-tied" I suppose.
Once I'd missed a couple days of school in 9th grade due to illness and the Phys Ed teacher assigned me some health paperwork to do since one can't "make up" missed gym time like they can take home a missed math assignment and work on it. I did the reading and questions perfectly and handed it over to her the next day. She turns to me, acting all suspicious (she was not a pleasant woman in general) and asked, "What did you learn?"
She wanted me to present facts from the assignment vocally, despite their written presence on the paper, as if to prove I'd actually done the work myself. It was all rather simplistic information, but 1) I'd already answered the questions; I hadn't *memorized* them, and 2) It was out of nowhere. I was put on the spot. So I completely froze. I think I managed to mumble something vague and she let it go, but even basic questions can leave me unable to speak intelligently if they're unexpected/out of the social norm.