Lots of people on Twitter think this happened at Glastonbury.
It happened a couple of years back in San Francisco at the annual Bay to Breakers footrace. In February 2009, city officials and race sponsors announced major changes to the race regulations. The regulations included an official ban on floats, alcohol, drunkenness and nudity. You can probably see why, although it doesn't seem to have worked.
However, lots of other amusing things have happened.