Driving on highway, brief hesitation as the vehicle switched from battery to gas operation followed by warning to stop the vehicle safely because the engine would shut off shortly. After pulling over and turning it all the way off and waiting a couple minutes, turned it back on and was able to drive, but with check engine light on. Took to dealership, who found that the transmission had failed. Strikingly, this was after only 31000 miles of normal use and consistent maintenance with the dealership. My personal suspicion is that there is a design flaw in the way the transmission handles shifting between battery and gas operation that places it under immense strain even in normal operation.