Source : Webster's Revised Unabridged Dictionary (1913)

Stumbler \Stum"bler\, n.
   One who stumbles.

Source : WordNet®

stumbler
     n 1: a walker or runner who trips and almost falls [syn: {tripper}]
     2: someone who makes mistakes because of incompetence [syn: {bungler},
         {blunderer}, {fumbler}, {bumbler}, {sad sack}, {botcher},
         {butcher}, {fuckup}]