For me, it's mostly things that reflect poor self-awareness. Talking way too loud, talking way too much, bragging, asking inappropriate questions, not taking hints, drinking too much, etc. Someone acting like that when you would expect for them to be on their best, most professional behavior means that their baseline is so out of control that that's the best they can do, or that they're not even self-aware enough to realize they need to tone it down.
The worst thing for me is when an applicant (or sub-i) acts too familiar with the residents, like we're all friends. You may be great and we may be great friends if you were in our program, but you're not in our program. On a related note, that's the best part about matching—you're finally part of the crew.