Having nothing to do. As much as work can be annoying or difficult or frustrating, there is nothing worse than have nothing to do all day. There is only so much surfing the internet and checking your email you can do.
Sure I try to get involved in other projects, but there are some days that are just dead. It would be the perfect time to just go home or chill in a park somewhere. Even just a small nap would make the time go by quicker.
Man sometimes you just run out of stuff to do.