This is impossible to narrow down to one or two, but these make my list
Lack of wirenuts on equipment ground splices
Equipment grounds not connected to boxes
Multiple wires on/in a terminal
Working space/clearance (keep repeating this about 100 times)
Oh, so that's what the green screw is for (as applied to panelboards)
Grounded conductors used for grounding conductors.
Few things are harder to put up with than the annoyance of a good example.