Years ago in this forum I remember seeing a deathtrap-hybrid of a bmx bike and a fixed gear. It was from a rider in Toronto, Shiznaz (RIP). Over the years I've lost the pics of his Haro, but never forgot the idea.
Flashforward to a donated Peugeot CPX200 bmx bike and some slow time at work. Despite the obvious flex in the seatpost, it hasn't started cracking yet after a few weeks of playing around on it. I've got a sissybar ordered to triangulate the saddle.
I've done alot of different riding on it already. This morning was my first time riding fixed offroad. Alot of fun, although some knobs on the front tire would have been nice.