Great thread!
I have many many things that irritate me at the gym and most of the time they are as a result of people not using machinary properly or making very obvious mistakes.
Again as already mentioned, walking on a steep incline whilst gripping hold of the handle bars on the treadmill. I once saw one girl/lady raise the incline to the max (15%) and up the speed so she was red in the face but she would not let go of that handle bar- her arms were literaly dragging her along and I wonder if she actually caused irrepairable damage to the ball and socket joints in her shoulders!
People rowing on the rower and not going all the way forward or back- I see BIG muscular men doing this- its as if they have forgotten how to fold their legs unless standing with a weight attached! Such a pointless exercise!
And one girl- choses any machine going, ups the intensity to the max and just goes so slowly you wonder if she has fallen asleep- the eliptical machine where your supposed to move at 130rpm, she is going at 80-90 (I don't normally spy on people but I was watching her to see if she was stuck/then in disbelief she could spend upwards of 20 mins doing this). I then watched her carry out the same method with every other machine- putting things to a resistance which is too much for her then going super slow. I reeeeeally want to say something to her as she looks rediculous but its not my role. I am a gym go-user and not a gym-trainer.
And one which got to me all of last year; the magazine reader!
Now this girl just make me LAUGH out loud every time I saw her! She'd arrive at the gym, no muscle, no tone, very tiny. She'd set up the stepper/stairmaster with her magazine on the face- always chose the same machine every time she was there. It would then be set to the max and we're good to go.
Her method was to step about three or four times, then stop, allow the machine to drop then turn the page....she'd have about 2 magazines to wade through- no pages were skipped, not even the ads, no more then 5 steps were done at a time, ok, maybe 10 if it was an editorial but she'd stop off any how to take a swig of her water. She'd have these huge 'Shure' DJ earphones on- obviously in the money to afford those, yet never once took up a personal training session which I really think could have worked in her favor.
See, when I or anyone else uses these machines, we go for it, and if she are thirsty, one hand reaches for the water, the other stabilises us and we drink (get this

Whilst we move at the same time!
This girl would do this same exercise every time I was there at the gym, obviously putting alot of effort into it but she never gained any tone (hmmmm I wonder why...). She was beginning to take on the spinning bikes as I left the gym for a better one, presumably because someone else took her machine (despite their being 4 of them).
People on their mobile phones WALKING on the treadmill- often when there is a queue. Annoying as they have yet to realise that in many (alas not all) of the gyms I visit, they are underground......(no signal) just funny more then sad! Seen it happen more then once.
My number one hate/mistake though is people trying to copy me.
I set myself a workout, I keep to it. I then do some floor work- I cannot count how many times I have started to do an exercise only to see someone else copy me. Now I sometimes take offence- this being MY workout and not theirs! I don't have a strict routine its based on how I feel. I am no expert, especially where floor work comes in, I must make many mistakes, yet someone will copy what I do for a good 20 mintes, from abs to a bit of yoga to back extentions to kettle balls, I turn around at every move, they (normally just one person) is/are doing the same thing. I have to say that when it really gets to me I either make obvious mistakes, then turn around and see this other person making the same mistake then point it out to them...see what they say, or I do some stretching- being very flexible, I tend to do far better and they tend to walk away at that point or I just roar at them FFS! STOP COPYING ME! and they walk away but basicly what I am trying to say: copying others move for move or even cardio machine for cardio machine- taking their stats and putting them into your own machine or aiming for the same levels- its just stupid. Everyone is an individual and copying someone elses workout without even knowing what their aims are is at best sad and at worst at risk of injury.