I've seen it where someone is using a series of three or four machines as a sort of "circuit" and then gets annoyed if you start using one that they're going to go to next -- as if they have the right to tie up more than one at a time.

Fortunately, the place that I use is the rec center at a local university. In the early AM when I am usually there, there are about zero students so it's pretty empty. And the same all summer at any time of day.