I have an example. In a previous role was in a position where we were negotiating a large-ish (>1M USD) multi-year contract when our client asked us to also do a proof of concept about some topic. It was just 1 week or 2 weeks so I said it would be free but because of various regulatory stuff we needed a contract in place. I was dealing with the same purchasing lawyer on both contracts. The process was super-standard…
What happened after? FWIW My wife and I were negotiating a commercial lease and we always did a diff between what they send back and what we sent them in case they pulled something like that. It may not be malicious (i.e. they updated a wrong version). They had integrity and they didn't do pull any stunts.
In spite of everything everyone on this thread thinks/says, it definitely wasn't an error. I spoke to another third-party vendor (over a beer) who were going through purchasing at the same time as us and he was doing the exact same thing with them.