This video is from Cairns, Australia, site of this year's Union Cycliste International Mountain Bike World Cup. In it, a spectator is seen riding a bike down the course and crashing violently, smacking his head several times on the ground before coming to a stop.
It's unclear how the fan got the bike—some outlets are reporting that he stole it, others say he offered to take an injured rider's bike down the hill for him—but he did not remain on it for very long.
There were two lengthy course holds, one for an injured rider, and one for an injured fan who decided to ride a bike down the track in a drunken state, and proceeded to crash about 10 feet later, knocking himself out.
This is the only report we've seen claiming that the fan may have been drunk, but multiple reports claim the crash caused at least a 30-minute delay in the competition while he was treated on the course. Reddit user perusername claims the fan was taken to Cairns Base Hospital and is being treated for a spinal injury.