I was thinking that the ultimate don’t-care-if-we’re-disqualified marathon has to be San Francisco’s Bay to Breakers. It’s a serious race that’s a pre-qualifier for the Boston Marathon, but it’s also a huge costume party! It starts in the Financial District downtown (where people toss tortillas in the air at the start), goes up the dreaded Hayes St Hill, passes through Golden Gate Park, and ends at the Great Highway where they have a costume judging area. There are bands along the way that play music for the passing runners.
One recurring ‘costume’ is naked (hence the nickname ‘Bare to Breakers’), but unfortunately most are not bodies anyone wants to see. There are also Centipedes (where groups of runners do the entire race in formation – there’s a minimum number required) and floats. Officially nudity is banned in the race (it’s not one of the festivals that SF has exempted from their nudity ban), but that never stops some people. Alcohol is banned and confiscated, but still shows up too. There are also groups who dress as salmon and run ‘upstream’ (from the Great Highway to the Financial District). Most just wear standard running gear and do not wear costumes – a large percentage of the runners never register.
The race started just after the Great 1906 Earthquake. It’s gone through numerous sponsors. Naturally, pictures are available on Tumblr and Flickr (some are NSFW).