This explains something I've seen with some of my minor-leaguers. I've released guys, only to have them show up on one of my minor-league rosters again later. At first I thought I'd just messed up (you know, "Gee, I thought I released that guy but I guess I didn't get around to it"). But it's happened to me three times now, and I'm not quite that absent-minded. So I've seen the same glitch.
The funny thing is that I ended up keeping one of the three guys. Between the time that I released him and the time he reappeared on my Class A roster, his ratings went up enough that he was better than some of my other minor-leaguers at that position. So one of them got the axe and he got his roster spot back.
If we start giving players nicknames, I guess I'm going to have to call that guy Zombie. He returned from the dead.