Going wayyyy too hard
This is a habit for a lot of guys who join a workout class, and that's why they're prone to more injuries. Apparently dudes feel uncomfortable when girls are more flexible with them in yoga, so they had to create a more masculine version of yoga called "Broga" because they felt so competitive and insecure. Lame. Regardless of gender, let's say there's a jump rope portion of the workout class. It is so annoying to be working out next to someone who is going really, really fast with the jump rope making an obnoxiously loud whipping noise and they can probably take an eye out or even a limb when they're going that fast. It's also kind of pathetic watching someone flailing their limbs and looking like an idiot while overexerting themselves. Or consider the fact that if you're going way too hard in the beginning, you will inevitably feel like you're going to die at the end and will be flopping around just trying to keep up. No one is impressed by someone that's working infinitely harder than everyone else!
[Photo via @rjathayde]