[petercooper]

I wouldn't usually just throw out a data point but since so many people here are saying they haven't heard it in the UK, I'm in the UK and certainly do hear "on accident" occasionally and have used it myself - it is less common though (An exclamation of "it was on accident!" is the most natural use I can think of - which fits with the idea it came via "on purpose"). Disclaimer: I am somewhat into youth culture and use a lot of slang in everyday speech.

(So I just asked my employees - who are somewhat younger than me. They had heard of it, claimed not to use it, and believed it was poorly educated people with a poor grasp of English who used it in their experience.)

[mziel]

Are you sure, it's not "It was an accident" ?