About the worst was the guy who set up shop to work on handguns, specializing in 1911 target guns. He took the sear engagement down until it was a nice light pull. But then his customers complained about the hammer falling to half cock when they loaded the first round. He ranted about the damage to his "perfect" sear, so he came up with an easy fix - grind off the half-cock notch.
Not long after, one of his guns did just what you think and ripped off 7 rounds rather quickly. No one was injured though a bullet narrowly missed the range officer. The "gunsmith" was visited by an attorney and a couple of police officers. It seemed that he had an FFL but had neglected small details like a state firearms license, business license, zoning laws, etc. He was persuaded to go into another line of business, and last I heard he was repairing lawn mowers.
I am not sure I want to have him fix my lawn mower, though.