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?