Not one thing, many things...
People who talk don't close their pie hole while chewing.
People who actually tell you how they are doing in detail when you say "hi, how are you" as you pass one another.
People who start a conversation with "Personally it doesn't bother me but..." and then proceed to talk about something that bothers them.
People who consistently brag about how they got an excellent deal on the best [insert useless item here] you can buy.