Why didn't Nixon erase the smoking gun?

Question

Given that President Nixon was bold enough to erase 18 minutes of the "Nixon tapes" before turning them over to the Watergate investigators, why didn't he also erase the "smoking gun," the conversation in which he directs one of his aides to tell the FBI to back off the investigation into the Watergate break-in?

Answer 1

One possibility is that he was suffering from paranoia and losing his mind. The second possibility is that he was so delusional that he didn't think he'd get in trouble.