There is a great chasm on these phrases in the US. The great divide seems to be currently centered at the age of 40. The younger generation has began shifting to "on accident" for unknown reasons. What is your view?
"On accident" (meaning "accidentally") does seem to be an unusual usage that frequently appears in opposition to the much more idiomatic "on purpose" (meaning "purposefully"). These are the kinds of idioms commonly used by e.g. children in explaining why something has gone wrong:
—"You broke my toy on purpose!"
—"No, it was on accident!".
A quick survey of the 34 incidences of "on accident" in the Corpus of Contemporary American English show about half have the sense discussed here, and "on accident" does occur in opposition to "on purpose":
HAAS: That happens in so many cases where you're got misinformation that's either leaked on accident or on purpose.
— from "Gunman Kills 32, Wounds 28 at Virginia Tech" on On the Record w/ Greta Van Susteren on Fox News, 2007
JACOBUS: Big difference when it's on purpose and when it's on accident.
— from "Dean, Democrats and Iowa's Deadline", on CNN, 2003
Other examples are from fiction:
"Dad better not see this or you'll get it. I'll tell him we were play fighting, and I slugged you on accident."
— Evan Shopper, "If I have to hit one of you, I'll hit you both" in The Massachusetts Review, 2003
She was thirteen years old, called herself a "gangsta ho" even though all her friends were white, and had already dropped out of school. "On accident," she said - she'd broken her collarbone the year before horsing around on her cousin's dirt bike and missed so much school that she simply never bothered returning.
— Emily Shelton, "From MEMPHIS (Short story)." in Chicago Review, 2003
"By accident", in contrast, has 1419 results, making it more than 100 times more common, and occurring not just in spoken and informal written English, but also in formal edited writing in academic journals, magazines, and newspapers.
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.
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
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.
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.
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.
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.