Factual error: In the episode 'I Only Have Eyes For You', during the flashbacks to 1955 James and Grace dance, and James later shoots himself, to the song 'I Only Have Eyes For You', which wasn't released until 1959.

Continuity: When Giles looks through the window of a door at the high school and sees the janitor yelling at the teacher, the janitor's lips don't match with what he is yelling.

Continuity: When Buffy is waiting in Principal Snyder's office, the 1955 year book slides off the bookshelf and lands open, but when Buffy walks over to pick it up, the book is closed.

Revealing: After Buffy has entered the wasp-filled school there's a shot of the others standing outside waiting for her. In the establishing long shot if you look closely you can see they used the same shot as earlier in the episode - Buffy is standing there with them.

Visible crew/equipment: When Xander opens his locker and the gross hand grabs him, if you use slow-mo when the shot is inside the locker looking out, you can see the white t-shirt of the man whose arm it is.

Continuity: Drusilla kills Kendra by cutting open her jugular artery and leaving her to bleed to death. When Buffy enters a while later, there is not a drop of blood anywhere.

Plot hole: In the flashback to Buffy in L.A., how did Angel get there with the front windshield of his car blacked out? Being a vampire, he obviously needs it during the day, but he also wouldn't be able to see traffic. And it just seems to be too much of hassle for him to remove it at sunset, just to reapply it at sunrise, just to remove it again, etc.

Continuity: In 'Becoming, part 1' as Buffy kneels over Kendra's corpse you can see the door over her shoulder, it stays closed all the time. There is a cut to a shot of Buffy from behind showing a cop raising his gun, but he couldn't have come through the door without us seeing him in the last shot.

Continuity: According to Giles in 'Revelations' there are 12 cemeteries in Sunnydale, but in 'Becoming Part 1', amongst others, Buffy agrees to meet someone (Angelus in this case) in 'the cemetery'. If there are 12 in the city, how would they know which one she means?

Other: When Willow is attempting the spell in the hospital, the subtitles state that Xander is speaking a line of the incantation. It is actually Oz speaking - Xander is off rescuing Giles, as we see in the very next shot.

Revealing: Throughout almost the entire sword fight between Buffy and Angel, from the far away shots, its clear that the actors are stunt doubles. It's very visible when there is a shot from the top.

Plot hole: Spike makes it clear when talking to Buffy that he wants Angel dead. Yet later on, when Buffy confronts Angel and says she doesn't think she can take them all on alone, Spike only knocks Angel down when he easily could have staked or beheaded him.

Continuity: When Joyce is being interviewed by the policemen at the house, you can see the number outside the house, but it says 163 instead of 1630, the number on which Buffy lives.

Continuity: In "Becoming Part 2" At the end when Spike chokes Drusilla unconscious, he picks her up, one of her hands is resting on her stomach and the other is hanging to her side. It cuts to a shot of Spike looking at Angel and Buffy fighting then to Spike leaving with his back facing the camera. You can see that the hand resting on Drusilla's stomach moved to Spikes shoulder.

Plot hole: In 'Becoming, part 2' Spike decides he doesn't want to see the world end and makes an alliance with Buffy. Later on, however, he helps Angel get the information he need from Giles to end the world. If he's trying to save the world, why help Angel destroy it even faster? He didn't need to buy Giles any time - he had already convinced Angelus to keep Giles alive for the time being. If he had truly reached breaking point later on Spike could have called in Drusilla then, but as it is he helped Angelus way too early and nearly cost Buffy her life. [Some people disagree at great length, others don't - I haven't seen the episode myself, and the explanation of the mistake and the corrections just make my brain melt. File this under "make up your own mind and just let it go"].