People who talk during lectures. It's rude - both to other students and to the lecturer. Its not exactly difficult to refrain from talking for around 50 minutes.
Furthermore, people who start packing their stuff away, rustling papers, or putting their jackets on 5 minutes before the lecture ends and therefore cumulatively making a noise which makes it almost impossible for others to hear what the lecturer is saying. I remember in school if people did this the teachers used to get these selfish dicks telt. Even if there was a minute of the class remaining before the bell went, the teacher would make them take their jacket back off, only for the bell to go just as they'd taken it off and they'd groan and grumble and have to put it straight back on again. This was extremely satisfying.