Revealing mistake: When Buffy is invisible and in the social office in "Gone", she is searching for the summer files, and you can see a thumb picking them up. (00:20:40)
Revealing mistake: When Buffy is at the desk of the social assistant you can see the white wire at the left of the keyboard. The white wire controls the fake keyboard: you can see the wire moving. (00:20:50)
Dead Things - S6-E13
Revealing mistake: In "Dead Things," after Buffy and Spike sleep together, when Spike asks, "Are we having a conversation?" and Buffy says, "What? No?" There is a side shot of her. If you look closely at her back (pausing helps), you can just barely see Buffy's skin colored tube top. Trust me, it's there. The time code is 1:22.
Revealing mistake: Right at the beginning of the episode just before we see Warren squirt Jonathan with water, we see Buffy on one of their security cameras. The footage of her is pre-recorded as the progress bar for QuickTime Media Player is visible at the bottom of the screen. The window was moved down to hide this fact, but not far down enough to hide it completely.
Revealing mistake: At the start of the episode where Warren, Jonathan and Andrew are on quad bikes chasing two vampires, a shot of the two vampires running towards the camera is used twice.
Revealing mistake: As Willow walks through the forest pursuing Warren, just after we first see Warren running, there's a shot of Willow walking into a bit of a clearing. Her shadow crosses a tree on the right (could be caused by the moon), but then a couple of seconds later another shadow is visible on a tree to the left - obviously an artificially lit forest. (00:33:15)
Revealing mistake: In the opening of the season 6 episode "Two To Go", Willow has gone crazy and is trying to kill the two remaining members of the Trio, so she is pulling the brick wall of the jail apart brick by brick. The problem is that if you look carefully, they are obviously not bricks being pulled away; they look more like plastic or plaster.
Revealing mistake: As Willow is raising the satanic temple from the earth, we can see the support cables holding the temple upright from behind. This is plainly visible in the bottom right hand corner, complete with bright orange clasps on the temple itself.
Beneath You - S7-E2
Revealing mistake: When Buffy throws Spike into the benches at the church you'll notice that as he is about to hit the floor there is a blue mat where he lands. (00:36:15)
Revealing mistake: When Anya sings her song about how she will become Mrs. Harris, she spins Xander's chair around after putting a blanket on him. When she is also sitting on the chair, and you see an overhead shot of them both, you can see a section of the stage floor rotating beneath them keeping the chair spinning.
Revealing mistake: As Buffy throws Spike into a wall after he bites Andrew, Spike's stuntman is visible in-shot. His hair is slightly different to James Marsters'.
Revealing mistake: When the ubervamp is at the door of the Summers' home, you can see a black string attached to the inside of the door.
Storyteller - S7-E16
Revealing mistake: When Buffy, Spike, Andrew and Robin go to the high school and find it decimated, Buffy is thrown to the ground and attacked with a hockey stick. On the second swing, a sound effect is heard and Buffy falls over. But the hockey stick never makes contact with her.
Lies My Parents Told Me - S7-E17
Revealing mistake: When Spike tells his mother that he's become a vampire, she asks him if he's drunk. He responds with "Little bit." Just after he says this, someone can be heard coughing off-camera. It could be his mother because she isn't supposed to be well, but it seems odd that the camera wouldn't be on her during this. And a cough wouldn't really be necessary for the scene.
Dirty Girls - S7-E18
Revealing mistake: In the opening scene, where Caleb picks up the potential slayer in his truck, there is a brief shot of the two from the hood that pans slightly to the right. In doing so it reveals the edge of a screen showing a fence and trees passing by, revealing that the car is in fact stationary and in a studio. (00:02:40)
Chosen answer: He had larangitis and because of the shooting specifications they had to have him shoot the scene with a bad throat, they then re-recorded his lines and dubbed them. At various points during the scene and indeed the episode you can see him touch his throat or rub it.