The Collaborative International Dictionary of English v.0.48:

Blunderer \Blun"der*er\, n.
   One who is apt to blunder.
   [1913 Webster]

WordNet (r) 3.0 (2006):

blunderer
    n 1: someone who makes mistakes because of incompetence [syn:
         bungler, blunderer, fumbler, bumbler, stumbler,
         sad sack, botcher, butcher, fuckup]