not intentional or deliberate: an unintentional omission from the list.

1775–85; un-1 + intentional

unintentionally, adverb

accidental, unplanned, inadvertent.
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World English Dictionary
unintentional (ˌʌnɪnˈtɛnʃənəl)
not deliberate; accidental: the killing had been unintentional

Collins English Dictionary - Complete & Unabridged 10th Edition
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Example sentences
They then tried to haul it aboard, unintentionally separating the tentacled
  head from the rubbery tail sheath.
If leaders cannot speak without unintentionally spreading alarm, they should
  not speak at all.
If they have done so, even if unintentionally, they must pay a price.
The parodies have proliferated precisely because the footage they're based on
  is already unintentionally ridiculous.
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