[stcredzero]

The Mythbusters tried to slam a car into reverse at speed. What they discovered, is that there are safety mechanisms to prevent this from happening in automatics.

http://mythbustersresults.com/episode84

[kelnos]

Not just automatics... my manual (2004 Honda) has an interlock that blocks the shifter from moving into reverse if I'm moving forward at > 10mph.