Seth godin has a few examples of sunk costs. I believe these examples better represent true sunk costs than some of the examples given here (such as the movie ticket).

For example, suppose you have paid 50 dollars for a Bruce Springsteen concert. You have searched long and hard for tickets this cheap. Suddenly, somebody offers you 500 dollars for the ticket. Do you sell it? The ticket is now worth $500 to you, and you would have never paid $500 for a ticket in the first place.