I had a school 3000m race when I started my sprint finish way too early with 2 laps to go, went all grey (apparently), fell over twice in the last 100m, staggered over the line in 3rd place (yeah!) then had to be carried to the infirmary - it took me over an hour to stop madly hyperventilating and to be able to move, and the coach thought I was going to die (he'd seen the same thing happen before). Oh yeah, and I wet myself too.
Although worrying, that perhaps wasn't as unpleasant, as blowing up half way in the '95 or '03 London marathons, and those perhaps weren't as bad as getting diarrhea in the '02 New York marathon (although I had a hangover for that, so it was really my own fault).
Still, if you're going to hit the wall, get it over with early, that's my race strategy.