I'd second that with some improper tightening of the lock ring which could allow the cog to untighten/tighten with every skid, eventually leading to hub thread/lockring failure.

When installing your cog, grease the threads, carefully start on the threads, then back out slightly to make sure you are aligned on the threads. Tighten pretty well but not gorilla tight. Repeat for lockring.

Ride the bike around the block. Give it a couple of skids. If you feel any movement at all, time to tighten everything down again; maybe get your girlfriend to help.

Another test ride; everything should be right as rain.