If she keeps telling lies, she will eventually slip up.
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The expression 露马脚 (to reveal the horse's foot) comes from a story about a woman disguised as a man at a festival — her bound feet slipped out from under her robe, exposing the deception. 马脚 now means any revealing flaw or slip-up.
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Think of a pantomime horse costume: when the actor's real leg sticks out, the illusion is ruined. 露马脚 = letting the horse's foot show = giving yourself away.