Make something up. Piece together the various details of different bugs you solved in your career or that you were vaguely involved in the solution to something reasonably impressive and coherent. If you are a skilled developer you will be able to make the story convincing. Unless you happened to rehearse your answer to that specific question beforehand, there is no way to reliably deliver a good performance on questions like these. You always have the plausible deniability of "oh, this stuff happened years ago, so I'm a bit fuzzy on the details."