According to this https://www.motorhowto.com/change-gear-to-reverse-while-driving
Essentially, nothing will happen because it is impossible to change gears to reverse while you are driving. This holds true for both manual and automatic transmissions. However, for older cars, what would happen is that the engine would stall and would act like it’s in neutral, which can be dangerous.
Curious if anyone has tried this with their own car and has any personal experience?