I guess DNFs don't count, so I can't include the half-mile where I PRed my first quarter and then dropped shortly thereafter. This was at ~8000ft altitude and recovery took rawther a while...
So I'll have to go with my first 50k walk. Three steps after the gun I was DFL (couldn't actually racewalk, just "walked fast," so all the real racewalkers were out of reach immediately). Stayed DFL the entire way--as soon as I got close to catching somebody, he'd quit (this happened two or three times). When the next-to-last guy finished, I had another lap (5k laps) to go, and the finish official told me they'd turn the clocks off at time X. No problem, I figured; I was hurtin' some, but not *that* bad, and I would beat the cutoff by five minutes, if not ten.
On that last lap, I started sobbing--not crying, just sobbing--because my abs were completely shot from being tensed for five-plus hours, and I couldn't take a breath otherwise. Covered the final 5k in 45 minutes, though I still beat the cutoff by a minute or so.
Found out later that I'd had an incipient case of bronchitis before the race. Walking six hours in the rain didn't help it, and a week later (in addition to still having trouble walking) I had six degrees of fever and was very nearly hospitalized.
But otherwise the race went very well. Great sport.