Are there people you would not tell if their fly was down?
Pretty much, if I know you, I'm gonna tell you. I don't get embarrassed by those things easily. At worst, it will create a awkward moment that we will laugh about later.
If I don't know you, well I'll probably just laugh at you. Maybe not out loud, but you can be sure, I notice everything. So an hour later when you realize you were showing your goods to the world, yep someone noticed. It was me. And inside, I was laughing.
There is ONE person I know that I did not tell that their fly was down. My boss. He stood in the parking lot a few weeks ago and carried on a long conversation about direct mail, all the while his fly was open. That is an awkward moment that does not get turned into a funny story later. It just gets turned into a badly written blog post.