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.
Life Serial - S6-E5
Revealing mistake: In Jonathan, Warren and Andrew's van, whenever they're watching something on the monitors fed from their security cameras, you can see that the footage is actually pre-recorded due to the fact that the footage is being played through QuickTime media player.
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: Near the end, the TV behind Andrew shows the periscope's view, but it doesn't match the movements he makes with it. (00:36:25)
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.
Once More, With Feeling - S6-E7
Revealing mistake: During the song-and-dance number "They Got The Mustard Out", if you look closely at the dancers, the man in green to camera right of the second row is horribly out of step with the music, and with the rest of the group. The spell makes everyone dance far better than they naturally would (Tara loses the clumsiness she often displays, for example), so it is a mistake to have someone under the spell dancing so badly.
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: 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)
Revealing mistake: After the demon throws Buffy into a wall, she crashes to the ground. As she backs away, you can see the blue crash mat she landed on in the bottom left of the screen. (00:33:55)
Life Serial - S6-E5
Revealing mistake: The entire scene where Buffy is experiencing accelerated time, shadows of trees move in the wind but never actually grow or shrink. Even over short increments of time, the sun can move shadows noticeably, and judging from the speed of the people around her, Buffy is in accelerated time for at least half an hour. (00:08:25)
After Life - S6-E3
Revealing mistake: When Dawn is possessed and breathing fire the camera changes and we can see the safety tube the fire is coming out of. (00:23:55)
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)
All The Way - S6-E6
Revealing mistake: In one fight scene, Buffy uses a car door to block a kick from a vampire. The window of the car breaks before the kick connects with it. In fact, it breaks while the foot is still in mid-air. (00:37:00)
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.
Once More, With Feeling - S6-E7
Revealing mistake: The knives Giles throws during Buffy's training are a very poor CGI job if you watch it in slow mo. All three knives Giles takes off the wall are identical, with a broad curved blade and a plain black handle. The first one has a transparent blade as it flies toward Buffy, then we hear it bounce off the far wall and it's way out of scale for the distance. The second knife has a diamond-patterned grip instead of a plain one. The third knife conveniently spins in midair for a second so Buffy can kick it out of the way. (00:30:35)
Once More, With Feeling - S6-E7
Revealing mistake: When Buffy is singing in the Bronze with Anya and Tara as backup you can see Tara bump into the pillar and then dust herself off. (00:41:45)
Chosen answer: Angel is saying that he is still jealous and Buffy recognizes that this is his way of saying he wishes that they could be together. That is why they both smile.