I hear many people use sentences such as:

"I fell down the stairs ON accident"

but this sounds horribly wrong to me. In this situation I would always use:

"I fell down the stairs BY accident'.

I think using 'by' is the more correct and/or proper but what is the grammatical explaination (if I am indeed correct)?
Hello Guest

I think you are right. Those people might think the antonym of "on purpose" should be "on accident". As for the grammar of "by accident", I haven't much to say. It is a translation of French "par accident" or Latin "per accidens", and has been used as an English colloation since 14 century.

I work in various school settings as a substitute and at all levels (including staff) I hear, " (It happened )on accident". It just grates on my sensibilities every time I hear it! I am of the mind that it should be "by accident"therefore implying via accident, not on purpose.
The grammar usage in this country is just whacked anyway. At least in German, there is no question as they have very clear forms and word usage.