The Collaborative International Dictionary of English v.0.48:

Mispleading \Mis*plead"ing\, n. (Law)
   An error in pleading.
   [1913 Webster]

Bouvier's Law Dictionary, Revised 6th Ed (1856):

MISPLEADING. Pleading incorrectly, or omitting anything in pleading which is
essential to the support or defence of an action, is so called.
     2. Pleading not guilty to an action of debt, is an example of the
first; and when the plaintiff sets out a title not simply in a defective
manner, but sets out a defective title, is an example of the second. See 3
Salk. 365.