Using benches or squat/power racks for things that don't require benches or squat/power racks when there are no vacant benches or squat/power racks
I told myself that when I started working out again I wouldn't lose my patience with people using up the squat/power racks for foolishness. I would not be
that guy that worships at the Altar of Squatting.
I think I lasted three weeks before asking someone if they wouldn't mind curling somewhere else. I lasted two months before unloading someone's bar that was taking 4 minute breaks between curl sets in the power rack.
It's fine if there's a whole bank of racks, but if there are 3 or less and the gym's busy...take the curling somewhere else.
Lifting (usually curls) either right beside the weight rack or in a main artery of gym traffic
The only exception I have for this is if someone's doing some real power sets for dumbbell shrugs or something. Dude's shrugging with 150+ lb. dumbbells, I can wait for him to complete his set.
They're by the rack in case they start to drop the things.
My personal ones:
People bringing brats into the gym. Even if there is a day-care thing, they really need to keep them out.
I tore a ligament in my knee a few years ago when a 6-year old tried to vault me when I was doing a lot of weight on a leg press. Injured my back pretty good a couple of months ago when another kid basically ran into me when I was doing deadlifts.
Plus, I drop a plate or two on a kid because I don't see them and there's going to be some pretty serious injuries.
Also, the aforementioned not cleaning up the equipment...just ew. I sweat a lot myself, especially on HIT days. I still take the time to wipe stuff down, even if it costs me a minute or so additional between stations.
People trying to reserve multiple pieces of equipment for too damned long.
Doing a rotation between, say, 3 pieces I can understand and if the gym's not busy and they're not high-use pieces that's cool.
But reserving 5-6 stations at 6:00 PM, when they're all important ones? Pistols at dawn.
(Not saying people that allow others to work in, I'm talking about the ones who reserve like 5-6 stations and lose their mind if someone touches their settings)
One of the things I look forward to once I come back home and have the ability to stay permanently is building my own gym. I don't care if I have to drive hundreds of miles to pick up the necessary equipment, I will have a quality gym in my house.