I have a lot of "worst" errors, but the "worst of the worst" happened back before GPS to a friend and his wife aboard their 35 foot steel cutter sailing way down the Mexican coast. They were on a broad reach, running near hull speed, making for a coastal inlet on a very dark night. His wife was driving the boat and he was below looking at the charts. He came up to see how things were going and she told him she was locked in on the high range marker. About the time he asked her if she knew there was a low-lying barrier island between them and the harbor, they went aground at six knots at near high tide on the beach of the barrier island.
To make a very long story much shorter, six days later, they were back in navigatable waters.
s/v Paloma, Bristol 29.9, #141
Slipped in Bahia Marina, easy access to Corpus Christi Bay and the Gulf of Mexico.