The Collaborative International Dictionary of English v.0.48:

Inadvertent \In`ad*vert"ent\, a. [Cf. F. inadvertant. See 2d
   In-, and Advert.]
   Not turning the mind to a matter; heedless; careless;
   negligent; inattentive.
   [1913 Webster]

         An inadvertent step may crush the snail
         That crawls at evening in the public path. --Cowper.
   -- In`ad*vert"ent*ly, adv.
   [1913 Webster]

WordNet (r) 3.0 (2006):

inadvertent
    adj 1: happening by chance or unexpectedly or unintentionally ;
           "with an inadvertent gesture she swept the vase off the
           table"; "accidental poisoning"; "an accidental shooting"
           [syn: accidental, inadvertent]

Moby Thesaurus II by Grady Ward, 1.0:

89 Moby Thesaurus words for "inadvertent":
   arbitrary, automatic, capricious, careless, casual,
   culpably negligent, derelict, disregardant, disregardful,
   distracted, distrait, feckless, gut, heedless, ill-advised,
   ill-considered, ill-devised, inattentive, inconsiderate, incurious,
   indeliberate, indifferent, inobservant, involuntary, laissez-faire,
   lax, loose, neglectful, neglecting, negligent, noninterfering,
   nonrestrictive, off-guard, overly permissive, permissive,
   procrastinating, reasonless, reflex, reflexive, regardless,
   relaxed, remiss, scamping, skimping, slack, slighting, slurring,
   snap, spontaneous, thoughtless, unadvised, uncalculated,
   uncalculating, uncaring, uncircumspect, unconscious, unconsidered,
   undeliberate, undeliberated, undesigned, undevised, unguarded,
   unheedful, unheeding, unintended, unintentional, unmarking,
   unmeditated, unmindful, unnoticing, unnoting, unobservant,
   unobserving, unplanned, unpremeditated, unpurposed, unreasoning,
   unreflecting, unreflective, unremarking, unrigorous, unstudied,
   unthinking, unthought, unthoughtful, unwary, unwatchful, unwilled,
   wrecking