Got to witness a 6.7 Cummins runaway in a Walmart parking lot today.
It's a memorable event. LOTS of noise and LOTS of smoke. The owner was more irritated than scared. Said he'd just gotten the truck out of the shop. It was in the shop for excessive oil consumption. My guess is an oil seal in the turbo was leaking and finally let go. It sounded to me like the engine blew; at any rate it was turning so fast I'd never trust it unless it was completely rebuilt.
Hope the thing was still under warranty.