accidentally

/ˌæksəˈdɛnt(ə)li/
/ˌæksəˈdɛnt(ə)li/

adverb

  1. In an accidental manner; by chance, unexpectedly.

    "He discovered penicillin largely accidentally."

  2. Unintentionally.

    "He accidentally exposed the bacteria to mold spores."

Synonyms: casually, fortuitously
Antonyms: advisedly, by choice, by design, deliberately, designedly, intentionally, on purpose, purposely