In 2001, Nigel Short went public with the opinion that he had in fact played Bobby Fischer on the Internet Chess Club.
This was denied by Fischer himself, and the consensus seems to be that it was not in fact Fischer that he was playing. However, though I have the impression of reading something about who Short was really playing some time ago, I cannot remember this for sure and now cannot find a conclusive source on who Short's opponent actually was. Has there ever been a definitive statement on this?