Everday stuff that drives me mad:

A: Have you heard of [book, software, blog post, trivia, news]?

B: No!?

A: Really? *raises eyebrows*

Don't do that. Ever. Don't ever fake surprise. It does nothing good. It damages and humiliates.

Look: The last thing people need is someone telling them (or making them feel) they are inadequate in a social context. The only thing this consistently does is to let you look like a douche or even bully. In the short run this might give you the feeling of power because you know something the other does not know. In the long run, it gives you a negative aura.

There is always more knowledge than any one person can possibly know. No matter what field or theme you are talking about. If you feel smug for knowing, consider what the other person may know - and you don't.

Take that exact opportunity to offer to explain it. You will be remembered to be helpful and positive.