These kind of convictions aren't rare in the firearms domain. People get arrested or convicted all the time for doing stuff they had no idea was illegal. More recently a Navy Sailor (Patrick Adamiak) in the pipeline to be a SEAL was convicted after selling an imported parts kit that had been destroyed per ATF guidelines from the early 2000s. But apparently when he resold it, (after buying it openly from gunbroker), the ATF decided the way the kit was cut up was wrong and put him away for 20 years. Oh yes they had a few other excuses -- he had a decomissioned RPG tube, so the ATF just put an entirely other gun inside the fucking tube and fired it to claim it still work.

In that case, even the Navy, which almost never does this to felons, terminated him with an honorable discharge and even let him run up all his liberty time before releiving him.