What is It About Doppelgangers? They Always Seem So Angry.
If you carry a knife, you risked being stabbed by your own blade. Most likely wielded by Evil You from Mirror World.
The spot was put together by AMV BBDO for the Tower Hamlets Council, which is concerned about "rising!" knife crime and convinced people won't stop carrying blades unless they believe they're disproportionately likely to trip, fall and impale themselves.
Bet that pansy-ass excuse didn't stop Genghis Khan, though. (Yeah, I know that's not Genghis, but it's a pretty rad picture.)