[p8952]

Seconded. I've never heard this in the UK from people of any age.

I wonder if it has anything to do with the way "an" sounds when spoken with an American accent. A mishearing and then repetition of "an accident" seems much more plausible to me than confusion with "on purpose".

[forthefuture]

"An" in American sounds like "Anne", so I don't think that explains it. As an aside I've never heard someone say "on accident" so it might just be stupid people. Another good example is "would of" instead of "would have".

[lstamour]

To be clear though, you could contract "would have" as "would've" -- the mistake there is again the mis-hearing as "would of", I suppose.