Half reps is definitely something that prompts me to teach them how to work out, but I don't because I know some people may not appreciate me being their coach.
The most annoying thing for me is when people put dumbbells away in the wrong order

how hard is it to put the 20's, 30's and 40's together? I hate walking around the dumbbell rack looking for the other 40 because some idiot doesn't know how to count!