It's not just bad apples. Sometimes people just aren't a good fit for a role. It turns out they don't have the right skills mix, need too much direction, etc. I've definitely seen perfectly nice folks who come in but it turns out they can't really do the job and no amount of coaching seems to help. It's better for everyone not to string things out too much.

Being honest, I've had one job I wasn't a good fit for myself. But the dot-com bubble burst took care of that one relatively quickly.