Over Processing
Over processing basically is the use of too many effects processors, using them at higher outputs than you should. There is a saying, "too many cooks can spoil the pie". Well in mastering music, the same thing can apply. There is only so much processing that can or should be done to a mix before it is ruined by somebody who thinks they are doing something impressive by using a boat load of effects to master a song.
First of all mastering music consists of only giving the music what it needs, not what you want to give it. This is part the mastering process where you just listen for problems an then begin to bring the audio up to the competitive industry standard of quality.
If you cannot hear for yourself why over processing is not a good thing to do in mastering music, then this is why you should leave it a professional with trained ears and years of experience. The trials, errors, and mistakes in life should be something that refines an improves a person for the better. Only someone that owns and acknowledges those things can begin to seek out the correct way of doing things if the passion is there to be better.