Continuity mistake: When Charlie is climbing down the cliff the rope comes undone from the tree stump, but a few seconds later you see it still tied to the stump. (00:12:00 - 00:12:45)
Continuity mistake: At the beginning of the episode as Clarke is walking towards the window in the door, her right arm is already injured, but she doesn't actually get that injury until she breaks the window and reaches through the glass, which cuts her right arm.
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)
Continuity mistake: During the fight between Anya and Clarke in front of the drop ship they are both covered in grime, and after Anya hits Clarke with the log Clarke's face becomes increasingly smeared with blood. Throughout the scene, you can see Clarke's face switch between grimy and bloodied at least a couple times, most noticeably in the shot just after Anya tackles her into the ground and the following one. (00:34:25)
Blood Must Have Blood: Part 2 - S2-E16
Continuity mistake: In the scene when Octavia and Maya enter the mess hall while trying to escape the guards chasing them, you can see Maya is walking in front of Octavia. In the shot immediately after, though, Octavia is the one in front and she is being stopped by a guard pointing a gun at her. In the following shot we are back to Maya walking in front still trying to cross the hall, just as she sees guards appear from around the corner; that is the point when the two turn around, and we are once again presented with the same shot of the guard intercepting Octavia, only from a different angle. (00:26:40)
Continuity mistake: When Clarke gives Lexa Anya's braid, we see Lexa accepting it with her left hand and then holding it with both. The next time she's on screen, though, you can see her holding it in one hand, still withdrawing from Clarke. In the following shot, she has the braid in both hands once again. (00:29:10)
