mine has happened with a 3500 and a 3001 both had the fail safe set to reverse.. but the thing is no power goes to the RX so its fail safe cannot kick in.. you also have no steering.. the light on the RX goes out and what every else is being powered.
there is a way of fixing it if you want a 2nd battery. It wasn't suitable for me but I did try it just to see if it would work.. run a little 12th scale battery to the RX and disconnect the red wire from the tekin to the RX.. this seemed to stop the problem.. not a good solution though..
cheers