I had my ignition key turn itself off (because of heavy keys) just enough to shut the motor off, in about 5 seconds I figured out what was wrong (still coasting in 4th gear) turned it back on which ignited the extra fuel still being sucked in through the carburetor which resulted in a huge bang and piles of black and brown smoke in the mirror and the explosion made a balloon out of my muffler (not as bad as Correaus's) but it didn't work to well after that to silence the rover.
Some "friends " of mine in high school used to do it on purpose driving through underpasses in downtown Calgary (with manual transmissions) automatics would not do it since the engine did not keep turning as you coasted with the key turned off. You needed a straight through type of muffler (we used to be called them cherry bombs,) if you tried it with a normal muffler it made balloons out of them.
I guess boys will still be boys nothings changed in 40 years if they are still doing it over in Briton.
Victor