I brace myself to be the idiot. I'm going to waste everyone's time asking questions that everyone knows the answer to, and I just got looped in, so everyone's going to feel like they need to walk through all the super-obvious stuff to satisfy the one guy who didn't do his homework.
So I start asking questions, and slowly begin to realize that nobody in the room has any idea what they are talking about. That there are fundamental misunderstandings and misconceptions about existing systems. And, naturally, it turns out that the questions I have are questions that other people have.
This has happened to me so often now that you would think the sinking feeling I get before I brace myself to look stupid would go away, but it never does.