when I was a kid I took a bmx and put a road bike fork on the front with a 700Xcc wheel, then road it around, seriously a bad idea. I couldn't stear, couldn't stand up on the pedals without falling. and it looked dumb as hell! I rode it for about 2 months.
another genious one was me and my dad building a custom low-rider bike, by chopping the top tube and shortenning the rear triangle, looked cool as hell, but I think we should have measured for crank length, cause one day while riding it I was going around a turn, slammed the pedal into the road throwing me over the bars and sliding on my knee, still have the scar.
then there was the backwards cruiser, flipped the rear wheel around, flipped the cranks, turned the seat around and the handle bars. did you know that bikes aren't designed to operate backwards? I do now...
oh and they were all singlespeeds, so yeah.... oh and the lowriders free wheel eventually locked up so against my will it was a fixed gear (not the best thing for a bike with bad pedal clearance).