This nitrous backfire may have only lasted a few tenths of a second, but we imagine that was far too long for the unfortunate Harley owner shown in this video.
We are guessing you are watching the last time this guy feeds a poorly running Harley nitrous, at least without a lot of flame resistant safety equipment. Amazingly the only negative effects from this brief brush with fire was a burnt air cleaner and a few weeks without eyebrows.