#10 Not knowing signal flow at the front end of the mixer
I see this quite often in churches, where mic inputs need to be moved, and no one knows what the signal path is. It is essential that you know the signal path of everything in the system, not just for troubleshooting problems, but to prevent issues in the mix. Do you know if all of your mic lines are good without hum, buzz, or noise? If you are called upon to add an instrument at the last minute, can you get it in the system, and to the monitor system without major wiring surgery?
Typically in the audio world the "A2" or "systems tech" knows the "ins and out" of the system. The church world rarely has that luxury. Most church techs have to be "A1 (FOH engineer, "A2", "systems tech," Monitor Engineer, audio janitor, all rolled up in one.
Do your mic lines pass through a patch bay? Has the patch bay been serviced since it was installed? Dust and gunk will find its way into the bay.
If you have Phantom power passing through the patch bay make sure you have all patching done before musicians have IEM's in their ears. Make sure you have the Main House fader down, so you don't send a transient, (pop) through the system, potentially blowing components in the process. (or musicians ears!)
Sooner or later a mic line, mic cable, input jack will fail in the system. The more you know about the input signal path before the mixer the better prepared you are to fix it and then mix it!.