[snake_plissken]

Whoa, what? You learn something new everyday on here. I have never heard anyone in the US say "on accident."

Does "on accident" even make sense? The preposition "on" in this case implies intent, so who is causing accidents on purpose? And if you are, then it's not really an accident, it's sabotage.

[gambiter]

I grew up saying it in Oklahoma. My wife (from Virginia) would always say, "It's BY accident, ON purpose." So just to mess with her I started saying, "ON accident, BY purpose", which sounds just as strange to my ears.