Scary moment at Indianapolis 500 practice on Monday ... Stefan Wilson was taken off the track on a stretcher after his car violently collided with a wall during a wild wreck.
The crash happened just moments ago at the famed Indianapolis Motor Speedway ... when Katherine Legge got into Wilson's ride in Turn 1.
You can see in video of the accident, Legge struck Wilson with so much force ... both of their cars skidded into a nearby wall -- with Wilson's vehicle seemingly getting the worst of the impact.