Of a mountain bike. A Marin county (California) mountain biker was found guilty of riding his mountain bike on Wilderness trails (note the initial-cap) and was not only fined for the offense, but also had his bike confiscated and had to walk 25 miles home. In cycling cleats. This despite there being no signs anywhere near the trailhead stating that the trails were closed to mountain bikes.
Read about his day in Federal Court here.
Federal lands designated as "Wilderness" do not allow any mechanical means of conveyance as the law, which was written long before mountain bikes were invented, is currently interpreted. It's my understanding that the original purpose of the law was to keep motorized vehicles off the trails, but the strong hiking lobby has helped officials interpret the law to not only ban dirt bikes and ATVs, but mountain bikes as well. Last I heard, even kayaks aren't allowed in Wilderness areas. I believe they're afraid water molecules may be damaged.
I'm all for conservation. Definitely. Those who know me know how sickened I get at sprawling development and environmental negligence. But I'm also for common sense and believe in reasonable sharing of public lands among all user groups. I have no problem keeping me and my bike off trails that are heavily hiked or ridden by equestrians. After all, riding on a busy trail isn't any fun anyway. And it's dangerous. But every year more and more trails all across the country are shut-out to mountain bikers and other recreationists because of near-sighted bureacracies and outdated regulations.
This is the world mountain bikers live in. No matter how many studies show negligible impact from mountain bikes and no matter how courteous and responsible riders become, there will always be large numbers of ignorant, selfish, trail users who will fight to keep us from enjoying the very same things they do -- wild places. Nevermind the fact that the best weapon conservationists have against development is an increase in the number of recreational users, there are those who would rather have fewer people visiting public lands than risk having to share the trail with someone who chooses to travel by bicycle.
This is why organizations like IMBA.com and local BBTC.org must be supported at all costs.