Wikipedia's article on the fifty-move rule states:
All of the basic checkmates can be accomplished in well under 50 moves. However, in the 20th century it was discovered that some positions of certain endgames can only be won in more than fifty moves (without a capture or a pawn move). The rule was changed to include certain exceptions in which one hundred moves were allowed with particular material combinations. However, more and more exceptions were discovered and in 1992 FIDE abolished all such exceptions and reinstated the strict fifty-move rule.
I have never seen an example of this, though I used to think the K, N, B vs. K ending was one. When is it possible to force a win in 50 moves or more, but not less than 50 moves?