My own pet peeve is the guy who runs the power to the load first and then to the switch.
Also high on my list is the guy who runs two different circuits into the same outlet box, UNLESS it is for kitchen counters, in which case my peeve is exactly the opposite. Our local building code requires that if you have two sets of outlets in one box in the kitchen that they be on different circuits to prevent overload. You are supposed to run two circuits to kitchen outlets in our town, one for each of the left-hand outlets and one for each of the right-hand outlets. Anyway, for other outlets, I'm really glad that I always check each pair separately, as I have more than once found one outlet off and one on after I turned off a circuit breaker.
We had a weird thing one time from a do-it-yourselfer who ignored code, in which running the microwave in the kitchen and an air conditioner on the second floor at the same time would trip the circuit breaker. This dolt also did not pigtail any of the outlets on any circuit in the house, which meant that if one outlet failed everything downstream also would be out of service. It was a real pain trying to find out which was the faulty outlet since it was really hard to tell in which direction the circuit ran.
Finally, a real trivial annoyance, but still.....the guy was a dishonest contractor (we only learned this detail after we had already moved in...

), he'd deliberately overbuy on his jobs, bill the client for the extra material, and then take the extra material home to use on his own house. All the outlets are dark brown while all the walls are white. I am gradually replacing them over time.