Maybe there's a bit of psychology in the mix somewhere. Let me explain (it won't be a very good explanation, because I don't know the technical terms, but I'll do my best!).
If I say something happened '*by* accident' it kind of lessens the personal responsibility for the accidental thing. It was an accident; an act of God; something which just happened. I didn't make it happen, the invisible accident-causing pixie made it happen. Not my fault, mush! <g>
If I say something happened '*on* purpose' it sounds more like it was something I meant to have happen. It was planned; I had a hand in making the thing happen.
If I were to say something happened '*on* accident', there's a kind of weird conflict taking place. It sounds like the accident was planned; was deliberate.
And finally, if I were to say something happened 'by purpose', it sounds like the purpose-causing pixie made it happen and not me.
I suspect this an incredibly crappy explanation, but maybe someone out there can make some sense of it.
Yvonne
