I hate it when, in a crowded situation, someone will just look at you and say, "hey!", to get your attention, even if they know your name. Then I look at them confusedly, and they say, "yes, you," and I still have no idea who they're talking to, because one of the people around me could have also looked confused, and they could have been talking to them, not me. I always seem to get it wrong - I either keep staring at them awkwardly without saying anything, or I respond when they weren't trying to get my attention at all!
Ah yes, and it's also awkward when I suddenly meet eyes with a stranger - when I've thought to myself "oh, look, a person," and they've hopefully thought that too, not "oh look, a weirdo," (there's so many ways to be considered a weirdo, "normal" is a very exclusive, specific group for most people) and I either mumble a "hello" because I forget to project my voice beyond a whisper, or I attempt a smile, and creep them out.