Car will not stay in Drive

My daughter was driving her Prius through a parking lot. When she suddenly saw a spot she quickly shifted from Drive, to Reverse, and then back to Drive without ever putting her foot on the brake. Now the car will not stay in Drive or Reverse. If you hold the lever in Drive (or reverse) the car will drive forward, otherwise, it pops into neutral. Any thoughts? The car has 200K miles and burns oil. She has moved on to another car and this one is sitting on the side of the house. It drove fine before the parking lot incident and I hate to send it to the salvage yard.