The closest thing I've done to a trail like that, was sort of a 45-degree field of shale, with about 1.5ft of trail in the middle of it. We'd never been there before and thought the trail might lead somewhere, and after 15 seconds we were on the shale. 45-degrees isn't as scary as a vertical cliff, of course, but if you lose your balance over the edge, you're essentially rolling on razor blades until you come to a stop. Up ahead we saw that, indeed, the trail transformed into one of the vertical-cliff specials. We all stopped, got off our bikes, spent about 15 minutes getting turned around (preferred method was three men to a bike) and then we simply went back the way we came. The whole thing was just no fun, not tremendously horrible, but just not any fun. I can't imagine how hair-raising the vertical-cliff thing must be.
