Ah, my darker curiosity coming through. As for me, I haven't done a role I regret, but there have been auditions I've looked back on and went "Eww, how on Earth did I think that worked well?" I have fun with less-than-stellar stuff, such as one where all I got was my lines and no context, and I could tell it'd be no good... so I hammed it up. My only regret with that was I didn't ham it up enough, haha.