googley, not all students are uncomfortable when they see their professors outside of class. I don't have a problem with it. I say hello, we chat for a few moments, and that is that. Probably most professors aren't uncomfortable with it. It has probably happened to them so many times.... Do you know what makes you uncomfortable about it? Are you afraid they won't remember you from their class so you aren't sure if you should say hi or not? If it was a huge lecture class I probably wouldn't say hi either, as I doubt the professor would remember the masses of students every quarter. But if it was a smaller class where the professor definitely knew me, then I would say hello.