Dave, another rider whose name escapes me (let's call him Chuck), and I were riding the Mary's Loop/Lion's Head trails, west of Fruita when Dave's rear hub developed a little problem. It was a Hugi hub, with the fancy "stairstep" drive, rather normal ramps and pawls. Those hubs were capable of withstanding massive loads of torque, due to the multiple drive points of the design.
Unfortunately, the design is prone to failure, if the lube in the hub is too thick, or dirt infiltrates the drive. And, that's what happened. Suddenly, Dave's hub was freewheeling in both directions.
We sat on the side of the trail, scratching our heads. We were 8 or 9 miles from the truck, so we didn't want to walk out.
I looked at Chuck's pedals and took note that he was using toeclips and straps. I asked him for a toe-strap. Then, I wove it through the slots in the big cog, and the spokes of the wheel, which locked the cogset to the hub.
Dave didn't feel confident that he could ride the trail with a fixed hub. So, we swapped wheels, and started down the trail. It was an odd ride; I couldn't coast, of course, but I could shift gears. I was a bit slower than normal (hard to imagine), but the ride was actually pretty fun.
Dave and Chuck rode on ahead, when we got to the frontage road, and I spun along in a medium gear until I met them coming back in Dave's truck.
Since then, I have had a couple of single-speed fixed-gear mountain bikes, and I have enjoyed riding them quite a bit. But, I don't know that I would have ever even tried fixed off-roading, if I hadn't ridden Dave's wheel out, that day.
A little bit of good comes from every difficulty.