Bete Noire - S1-E16
Corrected entry: When Kate pretends to be Abby, trying to trick Ari, he deduces that she's not upon seeing her gun in its holster, and doesn't pick up on the more blatantly obvious fact that her manner of dress doesn't match Ducky's description of Abby as a Goth.
Correction: Ari doesn't see Kate until he looks and sees the gun. So he can't tell just by her voice.
Ssiscool ★