Every time I think I have seen it all I see something new. There are too many to list. But what I do see otherwise competent HOs screw up on is wire nuts. They need to be tightened. Just because you put three turns on it does not mean you are done tightening it. Loose connections = heat = fire. I've been on more than one call where the HO replaced a switch with a dimmer and weeks later an arc developed from poorly tightened wire nuts melting everything in the box. One HO called the fire dept and and was lucky to still have a house afterward.