All the time mate. I've come to realise about myself that I tend to want the underdog to win (by 'underdog' I don't necessarily mean the betting underdog) and, before the fight, will convince myself that I see the fight going that way. For instance, I wanted Cotto to win against Pac. I wanted Pavlik to KO hopkins.
What I do is bet against what I want. Money on Pac, money on Hopkins etc. I tend to clean up.
