What, in your mind, is the worst single performance at a major championships by anyone? For me, I exclude races where the "choker" runs hard and loses to an underrated runner (most of Clarke's big races). I'm talking more about disastrous tactics or just an obviously pitiful performance. BTW, in my own little running world, I've phoned in more than a few efforts, for which I'm ashamed. Still, I think it's an interesting topic. In no particular order:
Coe, 1980 800m - Well known debacle, Coe, at least 1 second faster than anyone else, completely out of position, scrapes by Kirov for the silver. I don't think Coe on the whole was a choker, but this was certainly a choking race.
Clarke, 1964 5k - Have only seen part of this race, but Clarke, who set the world record the next year, all but admitted he phoned this race in and didn't even really try to make an effort. Lost to a whole bunch of people who were not as good as he. I don't count the 10k, as Mills and Gammoudi were obviously underrated and Clarke at least got the bronze.
Mamede, 1983 10k - I don't count the Olympics the next year as maybe he had some issues I don't know about. However, I've seen this race. Mamede, who dominated the heats and was the fastest on the clock, ran 10 meters off the pack almost the whole way even though the pace was slow, was completely dropped with about 5 or 6 laps to go, then storms home and almost catches Mark Nenow, who really had no hope of medalling but battled the whole way. The thing that gets me is that Mamede obviously had a lot left, considering how fast he finished. People say he was a good racer when there wasn't rounds, but I think he didn't have it mentally that day.
Moorcroft, 1982 Euro 5k - 10 seconds faster than anyone, should have owned this race, completely loses position with 500 to go and settles for 3rd. I heard he was sick, but there were a lot of people he beat in this race (at least 6 or 7 people) who were in much better position than he entering the final lap, so I think it was a tactical choke.
I don't know anything about sprinting, but Powell's performance today brought this to mind.