This is when an impersonator has an almost perfect disguise, only to ruin it with a seemingly inconspicuous mistake. Perhaps the impostor blurted out something out-of-character (or a personal catchphrase), forgot to include a Distinguishing Mark (often an Intimate Mark), or accidentally revealed that they're left-handed. This can more easily occur during a round of Bluff the Imposter, and can be a sub-trope of Spot the Impostor except that their target generally has no advance knowledge that there is an impostor in the first place.
Related to Pull the Thread, as this is often what leads to it. Compare Saying Too Much, a more incriminating version. See also Conviction by Counterfactual Clue, when this gets even more unrealistic, and Conviction by Contradiction, which overlooks that there may be a perfectly reasonable explanation for the discrepancy. Often overlaps with Imposter Forgot One Detail. Compare For Want of a Nail.
See also A Glitch in the Matrix or a Revealing Continuity Lapse, for when dreams or illusions are given away by their imperfections, I Never Told You My Name and I Never Said It Was Poison for when the impostor gives themselves away by revealing they know something they shouldn't, or Confronting Your Impostor, which is what happens when somebody stumbles upon somebody else pretending to be them.
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