I've always been fond of pointing out one of the most dangerous things in personal aviation is a private pilot with a #2 Phillips screwdriver. That's because I've occasionally come across the aftermath of an owner who tried but failed to accomplish a simple inspection or maintenance task without the proper knowledge, tools or training. Too often, the results demanded a "real" mechanic to come in and fix the mess. And, while I've tried to do as much preventive maintenance on my airplane as I can, I've often gotten in over my head and needed some professional assistance, too.
This is a companion discussion topic for the original entry at https://www.avweb.com/features_old/owner-performed-maintenance