Blood Must Have Blood: Part 2 - S2-E16
Revealing mistake: When Octavia storms into the lab where the Mountain Men have been holding the Ark's people, she has her sword in her right hand. As soon as Kane speaks she makes to sheath it, and we hear the sound we're used to expect to accompany that motion, but you can see she's just holding the unsheathed sword behind her. Then she proceeds to drop it, hiding part of the action from the camera with her body, and although you don't see her catch it again that is made evident from the way she eases the sword on the floor beside her right foot a moment later. Furthermore, in the next shot her sword is not in its sheath. (00:30:05)