I buggered the snot out of several threads on a J-Red 70E crank trying to pop the flywheel by whacking the nut on the end of the crank stub method. Didn't have the nut threaded on quite far enough, hit it off-center, and it skewed badly down the threads.
Took it to an old school NAPA shop near me that does a bunch of serious machine work to see if they could bail my ass out of a really stupid (and potentially disastrous) screw-up since those cranks just don't grow on trees. The shop owner looked at me and said, "left hand Euro threads, yeah, right. Yeah got ten bucks? I'll see what I can do."
He grabbed what looked like a piece o' string, wrapped it around the crank and just started slowly chasing the threads from the good area to the bad area then back and forth until he was satisfied. Spun a new nut on it and I was on my way. Called the string a 'millimeter file'.
Whatever it was, it was slick and saved my butt!