When the holes go into the chamber, it seems that the cases will blank out discs into the hole until the last one won't quite clear, and the case doesn't want to extract. (Saw this once.) The other thing that I've seen is that the poorly threaded hole won't retain the screw, and it pops up until it won't go further. (Saw the remains of a couple of those, after the fact, in a junk pile.)

USUALLY, nothing happens because the driller has a clue or a conscience (generally not both!) and the barrel is scrapped.

There is the rare instance where the perpetrator has caught his mistake, run a set screw into the hole, dressed off the bottom flush in the chamber, and continued on mounting the scope. Haven't seen it myself, but it was related to me by one of the 'smiths that "trained" me many moons ago.
Information
Warning: This is a relatively older thread
This discussion is older than 360 days. Some information contained in it may no longer be current.