2 wires wrapped around a device screw.
This is not good because the terminals are only designed to accomodate one wire. I have seen 2 just laid one over the other and also 2 twisted together and then wrapped around. Proper contact is less likely with these methods.
Sub-panels with grounds & neutrals interconnected.
This allows neutral current to divide and flow on the grounding conductors and probably the metal piping systems of a building that are connected to the grounding system.
Replacement of 2 wire receptacles with 3-wire, where there is no ground.
This provides a false sense of safety. Also, if a ground fault occurs, the ground wire and the metal frame of any 3-wire equipment plugged into the receptacle will become energized, likely hurting or killing someone.
Insufficient conductor lengths in boxes.
This makes it difficult to properly install or replace devices.
Conduits & cables not supported properly.
When I climb up them, I need enough support.
Overstuffed(OK for a deli sandwich, not an enclosure) boxes.
Over heating of enclosures and increased likelihood of a fault occurring while trying to put a cover on, if you even bother to try.
Derating factors not applied, particularly to bundled cables.
This causes overheating of the conductors because air can't circulate.
Working clearances not observed. (Guilty, sir).
Try squeezing between a steel I beam and a control enclosure to take voltage readings. If your back doesn't come in contact with the grounded column, your big a** will.
[This message has been edited by Redsy (edited 03-06-2002).]