Suppose you go to see a movie. You bought a ticket previously for $20 and you have another $20 bill in your pocket. When you get to the cinema you notice you lost the ticket. Will you buy an other ticket and go see the movie anyway?
Most people answers no. (I won't pay for the ticket twice!)
Suppose you go to see a movie. You have two $20 bills in your pocket. When you get to the cinema you notice you lost one of the $20s. Will you buy a ticket from the other $20 and go see the movie anyway?
Most people answers yes. (Pity I lost a $20 on the way, but I came to see a movie.)
If you answer yes in both situations the end result is the same ($40 is history and you've seen the movie). Nevertheless, the average person comes to a different decision due to the circumstances. Our brain works in a funny way, doesn't it? :)
Here's a video of Dan Gilbert from TED explaining our behavior in such situations: