Rocky Falls, Mullions Range SF, SL10, M55, 1st. Tentative start through the pine forest and then started motoring. All going well till just on halfway when I tripped over a vine or branch and went flying. As I went down my right foot twisted out. Hopped/jogged/walked the rest of the course in pain (almost as short as going directly back). Couldn't effectively drive off, or even really weight bear, on right foot. Had a lead of 5:22 when I fell, which eroded to a narrower margin of 2:27 by the finish - I was helped by the big mistakes and injury-riddling of my competitors.
Verdict: sprain of the joint between two of the bones on top of my arch. Nothing obviously broken. EDIT: it was broken. Photos