Being petty is fun and sometimes very necessary. I always say this, but the petty card should be played only when necessary to send a contundent message that won't land any other way, and I think this is the perfect example of that.
Now that I think about it, being decently petty is just another way of being maliciously compliant. In this case, if the manager needs him to stay for an extra two weeks, he now has to rehire him.
Managers generally undervalue us as employees, but we have to keep working for them because we don't have anything else lined up. That is, until the day we can finally jump into a better position and say goodbye to those daily hours of being underappreciated.
Sadly, a lot of managers think we won't be able to find anything better on the market, and as a result, they take us for granted. I know sometimes it can be hard to tell, but they are human, just like us, and they tend to get triggered and offended when somebody decides to jump to another opportunity and leave them behind. So they can react just like teenagers, saying something along the lines of : 'Go then, go with your new best friends!' Not really understanding what that really means.
If they don't accept our very kind prior notice, then they should accept that they won't get the reports we were supposed to write the next few weeks. The question is, have they ever known what we provided for the company in the first place? Or are they finding out just now?
Great opportunity to say: 'Too late! I found a better position now'. Some people really only value someone when they lose them, huh? It was always true.