The 2021 Subaru Forester had been functioning normally with no warning lights. About 25 minutes into the drive I had come to a stop light on an upward incline. When the light turned green I moved from the brake to the gas as usual. However the auto-start function on the car failed, causing the engine to stall. A loud clunking sound was heard and the dashboard lit up with multiple lights, although I do no recall which ones were illumined. I attempted to press the gas again and the car did not respond. During this short time my foot was pressing on the gas the car rolled backwards and collided with the vehicle behind me. I continued attempting to press the gas pedal and still no acceleration was made. After parking and restarting the car it has resumed normal function. I have not yet visited the dealership so this issue has not yet been reproduced, nor has the offending system been inspected. I'm summary, the automatic engine start failed at a critical location, putting myself and others in serious danger and ultimately caused a crash.