Safety in general...
As others have stated, muzzle control, dangling etc. They usually think the default position for the trigger finger is on the trigger. They forget that a string of fire is not complete until the weapon is safe and holstered. They finish firing, lower the weapon and turn uprange to get more "clips" or load more "bullets" or just discuss why their gun isn't "good" because it didn't shoot where they aimed it.
Never had one look down the barrel to do a chamber check but there's still time
God love them, and I do too. I love teaching newbies but DANG you have to watch them like a hawk! And they are FAST too!