Revealing mistake: In Xander's dream, when he climbs into the back of the ice cream van, for a split second before the window goes out of view the backdrop cuts out and the blue screen is visible. Only visible in widescreen version.
Revealing mistake: When Glory creates the demon snake thing, we see a CGI image of the creature, but Glory and Buffy are nowhere to be seen. When we are shown where they are standing, their position should have been visible when the snake demon was created. (00:27:05)
Revealing mistake: When the porter is dragged back onto the train at the start you can see (more obviously if you watch in slow motion) that there's absolutely nothing grabbing him, not even someone's shadow. (00:04:55)
I Was Made To Love You - S5-E15
Revealing mistake: When Giles pushes Spike into the cabinet at the magic shop, you can see Spike's reflection in the glass of the cabinet's doors. But Spike is a vampire and therefore should have no reflection.
Revealing mistake: When Buffy is walking alone into the desert, a man's legs are visible in the upper-right corner of the frame and remain visible for a second or two. (00:03:35)
Revealing mistake: When Spike is playing with the Buffybot in his crypt and has her throw him onto his bed to stake him, the shot of him landing on the bed and the Buffybot climbing on top him is portrayed by stunt doubles. The shot is then interwoven with another shot of the actual actors. But it's very obvious due to the poor editing.
Revealing mistake: When Spike, Xander, Anya and the Buffybot are attacked by vampires in the cemetery, the grass for the cemetery moves underneath the vampires' feet as if it were a carpet. The cemetery was a sound stage and not an actual cemetery.
Revealing mistake: Watch the shot of Glory as she is hit by the truck at the beginning of the episode (easier to see in slow-mo/frame-by-frame). She isn't hit by the truck at all. It goes right past her. A simple optical illusion used in filmmaking for decades.
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: 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)
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)
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.
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)
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)
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: 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)
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.