These two events happened on the same week, so it was inevitable that the penny dropped for once and for all. Everything is relative.
Situation 1. Bright, fresh graduate, with serious ambitions, she had been a student of mine for not a long time. Sure I had my assumptions of what she knows! We started to talk about national festivities held in the spring, like St. David's Day, St. Patrick's Day, and that of the Dragon. To tone down the overwhelmingness of the new input, I wanted to bring up something related that she is also familiar with. So, just by the way, I ask her about our national holiday, March 15 (in cultural embeddedness, July 4 would be the US equivalent). She turned red. It was shame that came to her, not the answer. She had no idea. Gee, I would never have seen this coming! I thought I asked the obvious. Thank god, I recovered from my shock quite quickly, explained briefly what happened on March 15, and moved on as if nothing had happened. In reality, it was one of those surprises I will not forget. I wanted to make her feel better but ended up reminding her what she lacked. Score!
Situation 2. It happened in a class held for IT professors at a leading university. We were about to start an oral comprehension exercise, namely, watching a series that I feared would be too progressive in content for this predominantly middle-aged group. So I decided to dedicate a longer time to the intro. We address the question of family and gender roles, then, when they already seem relaxed, I pose the question: Who can tell me some more stereotypes? Silence. Silence. And still, silence. What, haven't we already covered some? What is the difficulty, I didn't get. Then someone speaks up, Yes, I can, but what is a stereotype? Just like above, I had thought stereotype was a term that didn't need an introduction. But I was wrong; while I come from humanities where every second discussion gravitates around stereotypes, they socialize around algorithms, another key term many of us would googletranslate. Anyways, fortunately, I was fast to collect my humanities jaw dropped, sparing them from feeling IT stupid.
What is to learn from these? Count on surprises, be flexible, don't take anything for granted, and never make your students feel that they should have known something. They come to feel hope, not stupid.