Fan Casting Spotting the Thread in Sort by Tropes

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.