The Collaborative International Dictionary of English v.0.48:

Unerring \Un*err"ing\, a.
   Committing no mistake; incapable or error or failure certain;
   sure; unfailing; as, the unerring wisdom of God.
   [1913 Webster]

         Hissing in air the unerring weapon flew. --Dryden.
   [1913 Webster]

WordNet (r) 3.0 (2006):

unerring
    adj 1: not liable to error; "the Church was...theoretically
           inerrant and omnicompetent"-G.G.Coulton; "lack an
           inerrant literary sense"; "an unerring marksman" [syn:
           inerrable, inerrant, unerring]

Moby Thesaurus II by Grady Ward, 1.0:

45 Moby Thesaurus words for "unerring":
   close, constant, delicate, direct, even, exact, express, faithful,
   fine, incorrupt, incorrupted, inerrable, inerrant, infallible,
   mathematical, micrometrically precise, microscopic, nice, pinpoint,
   precise, refined, religious, religiously exact, rigid, rigorous,
   scientific, scientifically exact, severe, sinless, square, strict,
   subtle, sure, uncorrupt, uncorrupted, undebauched, undegenerate,
   undemoralized, undepraved, undeviating, undissolute, unfallen,
   uniniquitous, unsinful, unwicked