I regret joining a competitive guild in Guild Wars.
It probably doesn't stack up to some of the other posts in this thread, but once it got really competitive that game eats up rediculous amounts of time. We played for hours at a time nearly every evening. The lack of sleep and the lack of freedom I had were really starting to have a bad effect on me. I was lucky that there wasn't anything too important that I missed out on during that time, because it had the potential to seriously mess up my life. Just about everyone I knew thought that I should quit, but didn't tell me until I did it myself. I appreciate that; trusting someone to make the right decision is really good for their ego. Either that or they didn't think I would listen...well, whatever.
From now on I've sworn to only play with people who don't mind more casual gaming.
There are other things I regret, but that was the most recent one.