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.
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.