THESAURUS
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