The Collaborative International Dictionary of English v.0.48:

Unintentional \Unintentional\
   See intentional.

WordNet (r) 3.0 (2006):

unintentional
    adj 1: without deliberate intent; "my heart with unwilled love
           grew warm"- George Macdonald [syn: unintentional,
           unwilled]
    2: not done with purpose or intent; "an unintended slight"; "an
       unintentional pun"; "the offense was unintentional"; "an
       unwitting mistake may be overlooked" [syn: unintentional,
       unplanned, unwitting]

Moby Thesaurus II by Grady Ward, 1.0:

53 Moby Thesaurus words for "unintentional":
   automatic, blind, casual, causeless, chance, compulsive,
   conditioned, forced, gut, haphazard, ill-advised, ill-considered,
   ill-devised, impulsive, inadvertent, indeliberate, instinctive,
   involuntary, mechanical, purposeless, random, reflex, reflexive,
   snap, spontaneous, unadvised, unanticipated, uncalculated,
   unconscious, unconsidered, undeliberate, undeliberated, undesigned,
   undevised, unexpected, unforeseen, unguarded, unguided, unintended,
   unlooked-for, unmeant, unmeditated, unplanned, unpremeditated,
   unprompted, unpurposed, unstudied, unthinking, unthought,
   unthought-of, unwilled, unwilling, unwitting