Which mathematician's marginal note went unproven for 358 years?

Fermat's, written around 1637 in the margin of a copy of Diophantus: that no three positive integers satisfy a to the n plus b to the n equals c to the n for any n above two, and that he had a truly marvellous proof which the margin was too narrow to contain.

He almost certainly did not. He proved the case n equals four elsewhere and never mentioned the general claim again in thirty years of correspondence, which is not how a man behaves about a marvellous proof. The note was found by his son after his death, among the margins of a book he had filled with observations.

Andrew Wiles proved it in 1994, having worked on it in secret for seven years, using machinery from elliptic curves and modular forms that did not exist in any century Fermat could have imagined. His first announcement in 1993 contained a gap that took a further year and a collaborator to close - a very public failure followed by a very public repair. The proof runs to over a hundred pages and nothing about it fits in a margin.