Here is my list of common mistakes I see at the gym:
Stopping on the treadmill, while it is still on (usually to pick up a fallen cd player or something). Although, I must admit, it is rather fun watching them go flying.
Not using collars when you don't have the most steady of lifting techniques. I see kids who can't keep a bar straight, bench without collars all the time. Again, its amusing watching the weight slide off.
Not paying attention on a spot. I once saw a guy actually walk away from the girl he was spotting to go and talk with someone. Luckily I was close enough to go and help her when she got stuck.
Old dudes who hang around in the locker room for hours on end, scratching their balls, while completely stark naked. That is just wrong.
Not wiping down the equipment after putting your greasy head all over it. I hate having to wipe up another person's mess.
Not re-racking the weight. Probably the most annoying thing you can do in the gym. No one likes to go hunting for a stray dumbell, or stripping a leg press machine that is fucking loaded to the max.
Lifting like a pansy. I'm talking about the people who look at you weird when you deadlift more then 50 lbs.