I made the mistake of serial monogamy... Not knocking monogamy (because it's my personal ideal), but a combination of various factors (low self-esteem, lack of self-identity, repressed anger) can cause a person to do some pretty messed up things (like sitting outside, underneath a window to listen in on a conversation in a foreign language you only halfway understand...... I still can't believe I ever got to that point in crazy...).
It isn't the biggest mistake, I don't think, especially when compared to some of the other stories I've read on this thread, but it's definitely one that eventually can cause issues. I've had to take almost three years off from dating to find myself, and even though I think I've met someone that might be worth the time, the past still kinda haunts at times...