Things can get blurry. There are some things some gunsmiths won't even try, while some talented amateurs do them. IMHO, most "johnsmiths" don't get into trouble trying to fix guns, they get into trouble taking them apart just for the he!! of it. Then they lose or break parts trying to reassemble the gun.
Among the things I have seen most often messed up by "johnsmiths": Drilling and tapping for a scope, bolt bending, trying to drive out a barrel obstruction (often a boresnake) and making things a lot worse, trying to shoot out a bore obstruction with a live round, trying to pound out a shotgun barrel bulge, trying to crown a barrel, cutting through case hardening doing a "trigger job", failing to understand the need for sear reset when doing same on a rifle, beating on a slide to "tighten it", installing a buttplate or recoil pad.
Oh, and I almost forgot the destruction caused by folks who believe the ads saying, "All you have to do is polish the gun and use our cold blue." I have seen guns worth hundreds or thousands made into guns worth $20 by that method.
Jim