I only have one that I really consider a huge mistake.. manipulative/controlling nut with a crazy temper. Should have walked away a lot sooner than I did.
And there was a guy I dated for a few months right out of highschool who was pretty dumb. He was a looker, but really had no personality at all. No regrets though, that was only meant to be temporary and it was fun while it lasted... and I learned a lot about myself and what I want in the process. My standards changed a lot after that relationship.