Drive a 99 camry LE. I sometimes shift to neutral on down incline to save gas and then switch back to Drive after the road levels off. But today I accidentally switched into reverse from newutral - there was a thump and the engine light (and other lights) came on and the car stalled. Realizing what I had done I iommediately switched to D but the car still stalled. Hence I brought it complete stop, switched off the engine by side of road and waited for a couple of minutes. Then I started the car and drove home, everyhthing was fine - no warning lights. Did I damage the transmission in any way?
First of all, if you are going to do that, keep your hand off the selector button. there is supposed to be a gate to stop you doing that. if you had wasn't on the button, you need to get it looked at because if you can gone into park the motor and transmission would have jumped out of the bonnet at you in tiny peaces. Normally at least one part of the drivetrain usually ends up on the road if you shift into reverse while going foward. If the car drove home you might be ok, but what you did put a lot of stress on the drivetrain, you would have worn the engine bearing a bit more than normal too. I would get someone you trust to look it over, you may have damaged something like an engine mount. If you take it to someone dodgy they will tell you just about everything needs replacing.
Why would you shift out of drive anyway? It doesn't save fuel as the engine as an overrun setting which cuts the fuel to the motor, in fact it uses more. you lose some control over the vehicle as you no longer the ability to use the engine to brake or to shift to power if needed. you use your brakes more which means they will not last as long. and if it still has any factory warranty left and Toyota find out it is not worth the paper it is written on because of drivetrain abuse.
Shifting the transmission to neutral on inclines is not a good idea and it will not save fuel. If you want to save fuel, press the OD off button on long downhills and let engine braking control your speed and let the EFI fuel cut circuit save fuel.
Shifting into reverse while going forward and vice versa puts a lot of wear on the friction surfaces, even when going slowly, like backing out of a parking spot. Bringing the vehicle to a complete stop before changing direction will save a lot of wear and tear on your transmission. Your incident probably did not do any harm to the transmission, other than put the equivalent of thousands of miles of wear on it. The torque converter probably acted as a cushion to prevent catastrophic damage.
According to the Prius' manual -- trust me, I haven't tested this one myself -- the D/N/R mechanism will not allow a dangerous swap of direction to happen, even if you ask for it. One side-benefit of electronics and cyber-stuff: safeties can easily be thorough.
It is ILLEGAL in most states in the USA to do that, and it is dangerous in all states. There is no benefit in terms of mileage that justify doing something so blatantly dangerous.
I suspect you did nothing to the transmission, but only time will tell. If you want to improve your mileage decending a hill, simply lift your foot off the gas pedal. You will get as good fuel mileage as is possible, and the small amount of back pressure on the drive train will help maintain full control. It will aslo avoid shifting into R while doing 45+ mph.
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