<I think you're supposed to be holding out some hope for the underdog's success, which may disqualify me from your short list...>
My favorite is the swimmer from Equitorial Guinea who swam the 100m freestyle alone (his 2 competitors jumped the gun twice and were disqualified). He was having difficulty making the return leg (having trained only for 50m swim in a 20m pool).
At some point he was barely treading water, and then with the whole crowd cheering, he finished just under 2 minutes.
