This is off the top of my head some of the stereotypes and/or funny situations I've found at the gyms I've frequented over the years:
-Guys that only work their upper-bodies. You can usually tell them by no matter what the temperature in the gym they're wearing trackpants. There used to be a guy at the gym my friend and I called "Legs." He was about 6' 5" and literally only worked out his bi-ceps and chest. His back was convex it was so bad.
-Newbies. I admire and respect anyone who endeavors to make physical fitness part of their life. Gyms, for the most part, are intimidating, boorish environments. That being said, don't front. If you don't know what you're doing, ask the trainer; that's why they're there. Last spring I had to help a guy out of the leg press because he figured he could do the weight, and couldn't. Oh, and hitting on the really fit girl doing clean and jerk doesn't engender yourself to the regulars.
-Free advice guy. I'm so far from being a professional trainer or therapist it's not funny and that's why I don't give advice about equipment or exercises unless asked or the person is going to immediately hurt themselves. Plus, what works for one person might not work for another. However, there was a football player at my old university's gym that would walk up to people at random and give them advice about form, reps, different exercises, etc. One of my favourite nuggets of wisdom, "Yeah, I find the 2 1/2 hour workout to be best. It really allows me to hit all the stuff I want to get bigger and do my abs."
-Grunty McGee. Now that I think about it, same gym. Another football player who used to do Herculean amounts of weights on bench, squat, and deadlift and would literally shout at the top of his lungs when he lifted. Problem was he did this for every exercise. I will concede that that it was a mere grunt during wrist curls.
-Seriously serious guys. There used to be this huge (easily 6'4" 240lbs) Eastern European powerlifter I saw occasionally who had the most amazing trainer I've ever seen. The trainer was maybe 5'5" on a good day and was probably in his 70's, and I have come to believe, was part of the same cadre of people who prepped the Russian fighter in Rocky II. I came in early in the morning once and found this trainer and his powerlifter doing various Calisthenics. And I'm not talking windmills. I'm talking hoping on one leg while bent in half and trying to balance while some 5'5" guy pokes you in the ribs and yells at you in Ukrainian.