Continuity mistake: When Buffy is talking to Riley, she puts the scarf on her head, on his hand. He talks to her and says "Maybe I am the bad guy," and in the next frame her scarf is back on her head again, then off again in the next frame. (00:24:20)
Visible crew/equipment: After Tara is found in the Janitor's closet and brought back to her room as Willow & Buffy come in, there is a mirror to the left of the door. It has a scarf draped over it to prevent it from reflecting things it shouldn't, but the bottom right corner is still exposed and in it you can see glimpses of the crew moving around behind the camera.
Continuity mistake: Buffy gets a massive scar on her head from fighting with Adam. This scar changes size at least twice throughout the rest of the episode.
Continuity mistake: When Oz leaves and holds Willow close to him, the position of their noses changes depending on the camera angle. In the shots of OZ they're pressed together and in the shots of Willow they're not.
Where The Wild Things Are - S4-E18
Visible crew/equipment: While the house is becoming "Wild", Riley takes Graham down to The Initiative. Once the door closes behind them and the mirror returns to its original position, the shadow of a camera is reflected in the mirror. It's in the middle of the frame, on the left, over a book case.
Deliberate mistake: Every time the commandos are seen in the early episodes of this season (The Freshman and Fear, Itself) they wear ski masks to protect their identities. Starting in The Initiative, they stop doing this. The reason is that the audience needs to be able to identify the different commando characters, but it's still a discontinuity.
Visible crew/equipment: In the episode 'This Year's Girl', when the Scoobies find the demon in the woods that Adam has cut open you can see a boom mike operator in the background. (00:07:40)
Visible crew/equipment: When Giles, Willow, Tara and Buffy (in Faith's body) arrive at the church in the armoured van, a camera is reflected in the van's back window.
Continuity mistake: When Buffy and Riley are in Buffy's room, Buffy holds Riley's hand with both of her hands and says to him, "Is there anything I can do?" But in the following shot she is holding Riley's hand with only one of her hands.
Continuity mistake: In the scene near the end when Faith is putting Joyce's lipstick on in the mirror and taunting her, it is obvious that the make-up department forgot to put Faith's tattoo on her upper arm. It's obvious if you watch back and forth between the mirror and the shot of Faith standing and turning, there is no evidence of a tattoo that has been established Faith has in the previous season.
Visible crew/equipment: The demons known as "The Gentlemen" always float about two inches off the ground. In one scene, two of them are moving through a park, and you can see the light blue cart they are standing on. (00:32:15)
Factual error: In "The Initiative," Riley is looking for Spike with a heat sensitive camera and says "we've got a cold one." If Spike was at room temperature as indicated on the sensor, he would be indistinguishable from from the air and objects in the room. (00:37:30)
Other mistake: Just after Giles becomes completely blind, Spike gets up to get a book off of the mantle. When he does this you can, very briefly, see James Marsters reflected in the glass doors of the bookcase that was behind him, despite Spike being a vampire. (00:27:00)
The Harsh Light Of Day - S4-E3
Continuity mistake: When Spike finally gets into the area where the gem is and Harmony follows him up there, he puts on that necklace thinking it's the gem. When he realizes Harmony has the gem it shows him taking off the necklace. It cuts to another shot and then shows him again with the necklace, but minus the charm.
The Harsh Light Of Day - S4-E3
Continuity mistake: When Spike pins Harmony to the wall, his hands change from flat against her to grasping her shoulders, depending on the angle. (00:12:00)
Revealing mistake: Several times during Spike's escape from The Initiative, his reflection can be seen in the glass. Vampires aren't supposed to have reflections. (00:13:00)
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.