I read of one such case:
There was such an award in synchronized swimming at the 1992 Olympics, when Canada's Sylvie Frechette was belatedly awarded a gold medal after it was learned a judge made a mistake entering his mark into a computerized scoring system. The correct score would have left Frechette tied with the original gold medalist, Kristen Babb-Sprague of the U.S.
[verbatim; text easily found by google]