One of the random quirks of language that pops up. Fill in the blank:
JOHN
Who opened my mail?
MARY
Sorry. I did it __ accident. I just wasn’t paying attention.
If you said “on accident,” you’re very likely under 30 years old.
In fact, among Americans in that age group, it’s becoming more common than the traditional “by accident.” And linguists [don’t know why](http://www.inst.at/trans/16Nr/01_4/barratt16.htm):
> Finally, why “on accident” arose is also unclear. Obviously, “on purpose” may have played a role in supplying an analogical form (I didn’t break the window on purpose; I broke it on accident). But “by accident” and “on purpose” have existed for hundreds of years without one causing the other to change prepositions, and we don’t hear “by purpose,” so why did the change happen when it did and why did the change have the direction it did rather than the other way round (in other words, to “by accident” & “by purpose”)?
My hunch? Start looking at popular kids’ TV shows and see when they started using it.