People who don't put things back when they're done, causing others to have to spend extra time looking for equipment or use things that aren't quite right (like a stability ball that's too big b/c somebody has "hidden" the right sized one). Also people who get on the treadmill/bike right next to the one I'm using if it's not crowded and they could get on one where they're NOT right next to someone. Fortunately I don't use the treadmills/bikes very often since I'm usually outside.