Accidental double tapping of .500 S&W revolvers is apparently quite common. This video demonstrates …

A few years ago S&W investigated the phenomena. They discovered, as you have probably already guessed, that the double taps were the result of what was essentially bump firing. If the gun recoiled quick enough, while the shooter was still applying trigger pressure, the trigger would reset and then, when the trigger finger caught up with the trigger, another round would fire. S&W filmed this video to demonstrate that their revolvers were not faulty …

[ Many thanks to jdun1911 for emailing me the link. ]