Redsy covered what I feel are the most common, but I will add another one that is really common.
Failure to follow the instructions (this is a listing violation which just might be the #1 violation in WV).
No explanation should be needed, but if explaining to a non-electrician I'd tell them that equipment is tested under a very specific set of conditions. When you do not follow intructions/comply with listing, then you are operating the equipment under a set of conditions that it wasn't tested for. Many times, this isn't a problem, but since you don't know for sure, you are gambling with peoples safety.
Tom
