Is it correct to say "on accident" instead of "by accident"?

He tripped on accident.
He was eaten by accident.

Those are the forms I would use, if forced to choose between them. However, there is no grammatical difference between the two¹, and I chose those because they "sound right". I don't believe there is actually a rule which states which to use, but I could be wrong about that.

This matter, of course, is solved if we use the word accidentally.

¹ Yes, the second sentence is in passive voice, while the first is in active, but I don't think that's the root cause.

"On accident" sounds strange to me. "By accident" sounds better, but I can't understand how it could be much different from "On purpose", albeit very strange to say "By purpose".

A quick Google search of "fell by accident" versus "fell on accident" suggests that "by accident" is used about 99% of the time.

But with "tripped by/on accident" there are more results for "on".

I've never encountered "on accident" until seeing this question, so I think it must be something peculiar to the US.

In situations like these, I tend to lengthen out the phrase.

Replace "by" with "by way of a/an"

He tripped by way of an accident.

accident here adds information to the clause "he tripped"

"Purpose" is something thought out, perhaps written down, requiring more formal thought.

A synonym for "purpose" is "intent". He tripped by intent sounds better than "He tripped on intent". Therefore, I would guess that although we are more comfortable with "tripping 'on' purpose" I wonder if "tripping by purpose" is actually a more accurate phrase. In addition, "He tripped on..." could be followed by an object as well. "He tripped on a cat"

Sorry for the rambling.

Both sound reasonable to my ear, though I like "by accident" better. Doing some basic corpus analysis finds that "by accident" is much more prevelant. The Corpus of Contemporary American English lists 1419 occurrences of "by accident" vs only 23 for "on accident". A quick Google comparison gives 7,030,000 hits for "by accident" vs 1,020,000 hits for "on accident". Interesting to be sure.

Perhaps this is in reference to Barratt's paper on the topic, and language change in general. Changes like this do happen, and it isn't unusual for usage to be clearly defined by generations (consider the verb impact). One conclusion drawn here is that people are seeking parallelism with "on purpose", though it also concedes that it's hard to know for sure.

As the original question makes pretty clear, this is a change in progress (you see this kind of stratification by age all the time with changes in progress). In the earlier stages of such a change, you can't expect to find it much in writing, so results from google, etc, are going to be misleading. It's likely there is also regional variation here as well. To me "on accident" (raised in northern California) is normal, though sounds slightly less formal.

As for why this might be happening, I like ShreevatsaR's theory.

Not a trace of doubt in my mind that 'on accident' is WRONG, 'by accident' is correct, non-negotiable.

The writer must think about the true meaning of his words, and bad grammar is often indicative of inexperience, if not partial illiteracy. It is not possible to 'do' something 'on accident'. An accident is, by definition, something not intended, or the unintended result of something done deliberately.

And I am purposefully writing. It is no accident that I am writing. I am doing this on purpose.

One cannot trip on intent (unless it is lying in the street and you fail to step over it). You can do something WITH intent, or can do it deliberately, or do it intentionally. These three notionally synonymous phrases each have different subtleties of meaning.

I have some friends in the south (Louisisana) who always say "on accident" and think "by accident" sounds wrong.

My friends in the North (New Jersey/Pennsylvania) think "on accident" sounds wrong.

It may be a regional thing, but I don't have enough data to say for sure.