I consider myself to be a pretty rational person, and try to avoid the cognitive pitfalls that, as a human being, I am prone to falling into.  One logical fallacy that always seems to get me, no matter how much I try to banish that mode of thought, is the sunk cost fallacy.

This fallacy is the idea that you’ve already invested enough time/money/whatever into something, so to stop now would be a waste.  It has its root in a number of frailties, from loss aversion to simple pride. It is a fallacy though, because how much you have invested so far in a venture has no bearing on whether that venture is going to be a success or not.  If you wouldn’t have started it, it is wasteful to continue it, rather than the opposite.

Despite knowing full well the fallacious nature of it, whenever I am making a big decision, it always creeps into my head.  How about you, dear readers, what logical fallacy have you had the hardest time keeping out of your mind?