The way it happened to me was the bulk truck put diesel into the unleaded underground storage tank. The bulk truck was there when I was doing my fueling. The station caught the error just as I was leaving & had a tow truck take my vehicle to a mechanic for draining the contaminated fuel. My vehicle was just one of many that had to be repaired on the bulk truck company dime.
Fortunately in my case no damage was done to the motor. I shut it down as soon as it "took off". The RPMs went to dangerously close to redline. Could've thrown a rod. It was a pain. Took quite awhile to get it running normally again but it was back on the road that night.