I did this one time on the motorway when it was dark and in a blackspot because the car behind me was flashing his headlights at me (i couldnt tell it was a marked 5 series)... I thought he wanted a race, so i put it into 4th and got up to about 110 or so before he put the lights on.
Pulled me over and said "Do you know what the fvck you just did?"
I told him that because he flashed his lights i thought he may have been trying to tell me something was wrong with my car, so i wanted to make sure my accelerator pedal was working properly.
He actually started laughing, said it was the most amusing thing he had heard all week and let me go because of that fact, after he checked out my insurance, and all of the other vehicle details of course, i almost peed my pants!