One problem related to engine shut off without warning has been reported for the 2007 Toyota Prius. The most recently reported issues are listed below.
At approximately 65mph a red triangle warning light appeared on the dash panel accompanied by a yellow circle. The parking button was flashing and a pink cartoon car appeared on the display. The car was running fine so I continued till I reached my freeway exit. At the bottom of the exit I had to stop for a red light. The exit is dark and has a sharp curve. Upon stopping the car the transmission locked and the engine shut down. I was unable to start the engine and and the car could not be pushed out of the way due to the transmission lock. Frantically I had to get a flashlight and wave off oncoming traffic. It was very dangerous. After a few minutes I retried the engine and it started allowing me to get off the ramp and get home. The warning lights stayed on. I took the car to the dealer the next day and after they tested everything they said that there was nothing wrong. Next time I might not be so lucky.