The Collaborative International Dictionary of English v.0.48:

Misreckoning \Mis*reck"on*ing\, n.
   An erroneous computation.
   [1913 Webster]

WordNet (r) 3.0 (2006):

misreckoning
    n 1: a mistake in calculating [syn: miscalculation,
         misreckoning, misestimation]