I recall an episode of the Prairie Home Companion radio show in which Garrison Keillor told a similar story in his News from Lake Wobegon segment. A farmer had loaded a tank of manure--or maybe he had pumped out his over-full septic tank--onto a flatbed wagon, and proceeded to drive through town on his way to wherever he was going to dispose of it. It happened to be a holiday and a parade was in progress on Main Street, so the farmer just fell in line at the end of the parade, and the crowd mistook him for one of the floats. The farmer was having a good time waving at everyone and seeing them wave back at him, so that he nearly missed the place where the parade turned a corner. He turned a little too suddenly and the tank rolled off the flatbed... That's about all I remember.

Do you suppose Garrison adapted his story on this song? Or is this theme a common one in folklore?