Completely out of curiosity, I was wondering how often PIs have to deal with "bad hires" in their lab/group? I sometimes hear the PI/trainee relationship be compared to marriages, where half ends in a divorce. Is there actual truth to this? There are also posts like these that reinforce this:

Tales of postdocs past: what did I learn?

As profs, have you ever accepted grad students only to regret it later? Does this happen often and how do you deal with it? Are problems typically that they cannot do the work, or that their personality is just not a good fit? I think for the most part PIs I've met handle these professionally, but I do wonder how they actually feel about the situation.

How often PIs have to deal with "bad hires" in their lab/group? I sometimes hear the PI/trainee relationship be compared to marriages, where half ends in a divorce. There are also posts like these that reinforce this. Is there actual truth to this? I think most of the PIs (principal investigators) I've met have handled this situation professionally, but I do wonder how they actually feel.

Specifically: how often does it happen that professors regret hiring particular grad students? How is this typically dealt with? What is the typical nature of the problem?