#2 - The critic who never lands a hand. First off, using the "guy who curls in the squat rack" is a bad example. You try to gently and politely educate people that this is not the purpose of the squat rack, but sure enough, the next week, dude is in there impressing every one with BB curls with a ten on each end. This is not an "education issue" it is a "douche bag" issue. I am convinced that these are the same morons who stack a 25 on top of three 5's behind a couple of tens on the same rack pin and leave dumbbells scattered all about the gym.
I have come to the point where I just keep my mouth shut unless some one is doing something dangerously stupid and even then I tell them to "feel free to ignore me." For guys who have shown an open mind and a willingness to learn, I will give them form pointers all day long, (and alternatively, I am always open to input). As for the know it all, "don't tell me" punks, not even going to waste my time.