Wiktionary
a. accidental; 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; unintentional. adv. 1 accidentally; unintentionally; without meaning to; as the result of an event not expected, aided, or designed by one's will. 2 As the result of some undesigned, unintended, unrelated, natural, or random occurrence; as the unexpected and unforeseen result of formerly unrelated factors. 3 by chance; unexpectedly; as the result of an unrelated series of events.
Usage examples of "by accident".
She had discovered it perhaps by accident, perhaps with help from Seth the real Seth and she could only pray that Tak would never discover what she was doing or where she was going.
We uncovered it purely by accident shot off a blast-pattern pretty much like any other and when the spill stopped rolling and the dust settled, there it was but no one in the company was sure if it was the kind of accident we wanted to publicize.