fly in the ointment

fly in the ointment
    n 1: an inconvenience that detracts from the usefulness of
         something
    

83 Moby Thesaurus words for "fly in the ointment":
      abomination, atrocity, bad, bane, befoulment, blemish, blight,
      block, blockade, bottleneck, bug, catch, cordon, corruption, crack,
      crying evil, curtain, damage, defect, defection, deficiency,
      defilement, despoliation, destruction, determent, deterrent,
      detriment, difficulty, drawback, evil, failing, failure, fault,
      faute, flaw, foible, frailty, grievance, hang-up, harm, havoc,
      hazard, hitch, hole, hurdle, hurt, ill, imperfection, inadequacy,
      infection, infirmity, injury, joker, kink, little problem,
      mischief, objection, obstacle, obstruction, obstructive,
      one small difficulty, outrage, poison, pollution, problem, rift,
      rub, shortcoming, snag, something missing, stumbling block,
      stumbling stone, taint, the worst, toxin, venom, vexation,
      vulnerable place, weak link, weak point, weakness, woe, wrong