Wolverhampton marathon 2005 - got to about 23 miles and just lay down on a grass verge and vowed NEVER to run again.
I lay there for about 20 minutes then walked 60-70 minute back to the finish and drove 90 mins to get home, When I got home I'd already changed my mind and entered the Abingdon marathon that evening.
Abingdon was a success (3:24:48) and I said that's it for marathons, but I went back to Wolverhampton in 2007 and ran a pb (2:55:55).