Years ago I bought a used Colt Gold Cup, had a really sweet trigger when I dry fired it at the shop. Hit the range, loaded up a mag and it went full auto, dumped the mag and scared the hell out of me. Took to a trusted smith and "someone" (perhaps the prior owner) had kitchen-tabled the sear. It does happen.

After that whenever I have a gun bought used I expect it to go full tilt boogie and am pleasantly surprised when it doesn't lol.