I became aware of the sunk-cost fallacy one day when I bought a giant diet coke that tasted funny.

My first impulse was to drink it because I had paid for it, and because it was huge, and because it seemed a shame to waste it. I think I was too far away from the fast food place to complain.

Then I realized that I wasn't particularly thirsty (I'd been interested in the taste and had eaten the food I wanted to drink it with), it had no nutritional value, and I didn't get it for the caffiene boost. There was absolutely no reason to drink it, so I didn't. And felt pretty proud of myself for noticing how stupid it would have been to have drunk it.