I was there on one of the occasions that happened. Standing at the fence watching.
During the Burnout smoke show, the next thing we hear this huge Kaboom, then literally the Hellcat had disappeared into this huge cloud of smoke!
Leaving the NOS on while doing a Burnout is a terrible mistake. Once the smoke was gone, the race is delayed for clean up.
So you can only imagine how ticked off everyone gets!.
Using a small shot of NOS isn't the worst, but you ALWAYS have to be sure you and your set-up are properly done each run!