Title speaks for itself. Personally, I have quite a few but I'll only list what I consider to be the two biggest. 1. Poor muzzle/trigger discipline: Now I'm not saying I'm gonna flip out over this, especially if it's by someone who's relatively new to firearms nor am I saying that I haven't caught MYSELF slipping once or twice, but it's still a big pet peeve of mine. I don't care if the firearm is loaded or not, I'm of the opinion that if you develop the habit of handling an unloaded firearm in this manner, it's only a matter of time before you start doing it with one that's loaded 2. Preachy Wannabe RSOs: These are the people who aren't RSOs, yet try to act like one. NEVER fails to call you out on something you did that wasn't necessarily "unsafe" yet gives you a snotty attitude while calling you out on it. Almost always has some type of egotistical demeanor and never fails to make you feel like an idiot. Also, they make you wonder if they've ever heard of common sense instead of what they've read in a book as it relates to firearms handling. Honorable Mention: People who shoot often yet only clean their guns once a year.