How about all the twig high school kids that loiter around and bench 1, maybe 2 sets in an hour, yet screw around on the equipment the rest of the time they are there playing grab-ass....
Secondly, I would have to say the unsupervised kids. One gym I was a member at a few years ago would allow kids in if they were with an adult. Well, as you might figure the fat heffalopes who brought them immediatly ditched them once they made it past the front counter and headed for the fat-chick yoga room. So end result, the pool, sauna and courts would always be filled with rat-bastards all the time. Then you would get the brats running around the weight room fighting and screaming. Management wouldn't do anything since their mommas paid top fees to basically socialize the whole time they were there and show off their white spandex clad cottage-cheese thighs.
God I hated that gym. Great equipment, clean.... but filled with non-hackers.