Linguistically speaking, I tend to caucus with the descriptivists. I have no problem if someone wants to willfully split an infinitive. If someone selects a favorite preposition to end a clause with, who am I to complain? You may consider this a questionable position for an English teacher. Of course, I do spend countless hours correcting grammar mistakes (or "mistakes"). But I make corrections not so much because of some belief in a Platonic ideal of English usage, but rather because I have a responsibility to equip my students with the tools of upward mobility, one of which is an ability to produce correct (or "correct") English sentences with something approaching consistency. On a fundamental level, though, I am much more concerned with clarity than with unbending adherence to often-arbitrary rules.
It is with some dismay, then, that I find myself forced to admit a certain nails-on-chalkboard reaction to "on accident."
Have you heard people saying this? Do you say it yourself? I first noticed some of my students saying it, but I chalked it up to second-language interference or perhaps petit mal seizures. Then, I noticed some of my tutors--ostensibly smart-ish people--saying it. Today, I heard a character in a movie saying it--and not as if the screenwriter had incorporated the phrasing as a way of indicating the characters' relative lack of linguistic sophistication. Apparently, "on accident" has gone mainstream.
Why is this happening? Painstaking investigation--OK, a Google search--reveals that this very subject has been the subject of linguistic research. As mentioned on the Grammar Girl website, a linguist at Indiana State University found that whether one says "on" or "by accident" depends largely on one's age. Children under the age of ten generally use "on" while people over the age of 35 almost universally say "by"; those between 10 and 35 may use either phrasing.
So, there you go: People say "on accident" because they are younger and therefore idiots. The terrifying implication is that, as we proper speakers die out, the official acceptance of "on accident" will prove inevitable. Might as well just go hole up in my mountain cabin with my freeze-dried rations and a copy of Strunk and White. I'll hold out as long as I can.