The Watergate Tapes (lost 18½ minute gap of audio recordings of American President; 1972)
Former US President Richard Nixon.
Status: Lost
Former President of the United States, Richard Nixon, recorded many of his conversations, phone or otherwise, in the White House for his own records. While he wasn't the first president to do this, with Lyndon B. Johnson and John F. Kennedy doing it before him, Nixon's in particular were quite lengthy. These recordings would eventually be used against him during the Watergate Scandal.
Reportedly, over 18 minutes of the final portions of the tape were wiped over. There are various theories as to why this happened. According to Nixon himself, the tapes were wiped by accident by an associate of his named Rose Mary Woods. Nixon was harassed by various members of the media, with many claiming he deliberately erased the audio. It is not entirely known exactly why the tape was wiped.
This, however, has not stopped various conspiracy theory groups from using the wiped audio as "evidence". Theories of what the audio contained ranges from a deal with terrorists to war plans with the Soviet Union. The audio is more than likely just more incriminating Watergate material or insignificant phone conversations with his secretaries.