Before Away Teams became a thing there we had people "loaned" to teams to update our code for team projects. I remember during one of the code reviews I did for someone "loaned" to us involved him commenting out validation checks in critical systems that had legal and financial ramifications. When I asked him why he commented it out he said "Oh, my code didn't work with those checks in there so I removed them." I get that the code is complex and he doesn't have the full domain knowledge, but he didn't even ask why they were there and why certain use cases were not allowed for that part of the code.