It should be obvious why you shouldn't attempt to try this.
We recently saw someone attempt to find out what happens when you shift a car into reverse at 70 mph. The results were underwhelming, frankly. That's because the tester was piloting a Ford Fusion, a car that is officially idiot-proof. Yes, Ford considered that people would be stupid enough to shift the car into reverse at highway speed, which is why doing so in the Fusion will simply activate the backup camera, rather than, say, destroy the transmission. But what happens if you do the same in an older vehicle with a manual transmission?
Keen to find out, for reasons unknown to us other than a desperate need for Internet fame, someone did just that in an old Ford Ranger pickup devoid of lockout safety features. We've probably all thought about it and wondered what would happen, but someone was finally brave dumb enough to give it a try.
The result is predictably spectacular, as the pickup protests to be being forced into reverse while cruising at 40 mph and the tires unwillingly spin backwards, producing copious amounts of smoke in the process. All we can say is well done, mysterious fool. You now have 20 seconds of Internet fame and a wrecked transmission. We sincerely hope it was worth it.