That the rack of dumbells is always crowded because every dude besides me is doing curls in front of the mirror.
I've a variation on this in that there's a woman who has started working out when I do at the small corporate gym I belong to. She seems to insist on working out RIGHT NEXT to the dumbbell rack, even though there's plenty of space to do otherwise. Today it was just her and I and she could've set up a bit further away, but nope, right next to the rack is where she was. Incidentally she's also the one that seems to need the stability ball (there ARE others in the gym) when I'm using it.