It's taken me a while to figure out what my pet peeve is but, I've finally realized what it is- cell phones. To me, there is nothing ruder than picking up your phone when someone else is in the middle of a conversation.
Example: The other day I was talking in the car to someone. We were in the midst of our conversation when all of a sudden, I realized I was talking to myself because she had picked up her phone! I didn't realize what had happened until I heard her, "hello".