[mikeash]

> Why did you not realize that “this guy is worth it” until he already has one foot out the door?

There are plenty of reasons this could happen. It's a mistake, and you should analyze the situation to see why it was made. But that doesn't imply you should let people walk out the door just because you made a mistake.

You assume that anyone who comes up with this thing implies a particular scenario, when it really doesn't. It could simply be that e.g. an employee got a cold call from some friends who recently got some funding and is toying with the idea of switching jobs, and the higher salary is the one thing that really makes him think of taking the offer.