one time in a 5k xc race, i was on pace for 16:00 through 2 miles (i had run 16:04 the week earlier). So 2 miles in 10:16, i had a 75m lead. All of the sudden at 2.5 miles i started to get dizzy. I was passed right before 3 miles, and basically walked the last 200m, and i couldn't stand up for about an hour afterwards. Ran about a 3:45 last 800. Final time was around 17:00.