One more cause for you

Power! I've seen so many instances of this that I can't even count. Customer has problems with random lockups, errors, etc., brings it into the shop, works fine. Customer gets it back home and BAM! The problem returns.

We then give the customer a UPS to try, the problem goes away and they may get this funny light on the UPS (Site Wiring Fault).

It's by far not the only cause, but is one of the biggies here.