On a recent business trip, I accidentally put 10.2L of petrol in a brand new TDI belonging to Hertz. The fuel capacity is 55L. Therefore with a full tank it had an 18% petrol / 82% diesel mixture in the tank.

Car drove just fine back to the airport. No check engine lights, etc.

Now I look at my credit card statement and they have charged me an extra ~350 euros presumably to drain and re-fill the tank. I'm not disputing that... I earned it. What a bonehead move.

However, my questions are these: 1) How did they know? Do they somehow test for the presence of petrol? 2) My guess was that running 18/82 mix of petrol/diesel one time wouldn't harm anything. Am I incorrect?