WordNet (r) 3.0 (2006):

stumbling block
    n 1: any obstacle or impediment

Moby Thesaurus II by Grady Ward, 1.0:

27 Moby Thesaurus words for "stumbling block":
   bar, block, blockade, bottleneck, catch, cordon, curtain,
   determent, deterrent, difficulty, drawback, hamper, hang-up,
   hazard, hitch, hurdle, impediment, joker, mountain, objection,
   obstacle, obstruction, obstructive, one small difficulty, rub,
   snag, stumbling stone