By accident means unintentionally; by chance, as the result of some unforeseen happening or series of events. Unexpected; resulting from an event without the foresight, expectation, aid, or design of the person by whose agency it was caused. The preposition used with the word accident can be a touchy subject for some grammar pursuits. “By accident” is furthermore in opposition with “on accident”, this is a variation found almost exclusively inside the United States. It is grammatically on par with the phrase “on purpose”. One can either do something on purpose or on accident.