Sunk cost fallacy

A sunk cost fallacy is a fallacy where someone convinces themselves that they should continue doing something because they put in a lot of time and/or effort into it, even if they actually get very little reward out of it. For example, a person might watch the first six episodes of "Battlestar Galactica", but decides they do not like the show. However, because there was a sunk cost, six hours of time, that person tells themselves that because they already spent so much time on it, they might as well finish the show.
An example in the real-world would be pro-war Americans during the Vietnam War. Americans who supported United States' part in the Vietnam War believed that the US already spent so much money, so much time, and so many lives of veterans and civilians that if the United States were to let the communists win the war, then all the loss would have been for nothing.