1) By chance 2) By mistake 3) Circumstantially 4) In an unintentional manner 5) Not on purpose 6) Unexpectedly 7) Without advance planning 8) Without intention 
• (adv.) In an accidental manner; unexpectedly; by chance; unintentionally; casually; fortuitously; not essentially.
Ac`ci·den'tal·ly adverb In an accidental manner; unexpectedly; by chance; unintentionally; casually; fortuitously; not essentially. 
accidentally (ak' suh DEN tuh lee) Happening by accident; not intended or expected: 'It was decided that she died accidentally not by any intended cause.'