I was discussing my blog and other writing issues with friends last night.  Between sushi orders, someone asked about the use of "on accident" and "by accident."  I had never heard anyone use "on accident," and, because I always use "by accident," I assumed that was correct.  I was right, to an extent.  Unfortunately, my answer probably tells you that I am over thirty. 

The Washington State University list of Common Errors in English  states affirmatively that "on accident" is incorrect:   "Although you can do things on purpose, you do them by accident."

The Urban Dictionary also agrees with me (I think):  "on accident" is defined as the "improper usage of the phrase "by accident". Started in the United States, but it has started to spread to other countries like a plague. A terrible, terrible, grammar plague."  In case you are not familar with this site, Urban Dictionary "is a slang dictionary with your definitions."

Pain in the English addressed the issue, with many comments.  Most agreed that "on accident" is incorrect, but becoming more common.

Grammar Girl also discussed the subject.   She notes that Leslie Barrett, a professor of Linguistics at Indiana State University, wrote a paper about "on accident" and other changes in language.  "According to Barratt's study, use of the two different versions appears to be distributed by age. Whereas on accident is common in people under 35, almost no one over 40 says on accident. Most older people say by accident. It's really amazing: the study says that 'on is more prevalent under age 10, both on and by are common between the ages of 10 and 35, and by is overwhelmingly preferred by those over 35.' " Unfortunately, however, "Barratt found that there is no widespread stigma associated with saying on accident."

Tim W. at Mother Tongue Annoyances checked the OED to research the issue.  Here is what Tim said: 


So, to conclude, many people (including me) think that the use of the term "on accident" is incorrect and annoying.  Unless you want to be considered incorrect and annoying by many people, you should use "by accident."    In particular, it is probably not a good idea to argue that your client 's conduct was not intentional because it was "on accident."