Source : Webster's Revised Unabridged Dictionary (1913)

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

         Hissing in air the unerring weapon flew. --Dryden.

Source : WordNet®

unerring
     adj : 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}]