Continuity mistake: In the beginning, after the monk traps Moloch and closes the book, the positions of his hands gripping the sides of the book changes slightly between cuts. (Ex. His left hand is lower, then suddenly slightly higher between edits).
Deliberate mistake: When Willow goes into her dark bedroom, she turns on her lamp. Somehow, her tiny lamp illuminates the entire room and also casts a shadow behind her, even though it's behind her. Clearly, when she turns on the lamp, they also turned on some set-lights in front of her to make the room bright enough for filming. There's also something else that gives it away-when she turns on the lamp, the studio lights come on a split-second later - just enough that if you look closely, you'll notice the delay.
Deliberate mistake: When Buffy, Xander and Giles try to "delete" Moloch from the computer, watch the computer screen as she sits down. There's already a folder labeled "Willow" on the desktop. Buffy then types in the word "Willow" to search for the document (without clicking on any sort of search bar, so typing it would do nothing), then says something along the lines of "That's probably it" and proceeds to click on the "Willow" folder... that was already on the desktop. The entire scene makes no sense.
Continuity mistake: When Buffy and Willow arrive at school at the start of the episode, Willow is wearing very dark, completely opaque tights that look to be black or at best very dark navy blue. When Xander finds her in the next scene, which takes shortly after on the same day, she's suddenly wearing much brighter purple tights.
Continuity mistake: In the beginning of the episode, when Willow gets the email from "Malcolm" and says "He's so sweet! What should I write back?", she instantly goes from facing forward at the computer to facing sideways at Buffy between cuts.
Continuity mistake: After Buffy finds Morgan's brain, Willow pulls up Morgan's school file on one of the library computers without even touching the keyboard. One moment the monitor is on the desktop, the next it's showing Morgan's file.
Visible crew/equipment: When Buffy, Willow, Xander and Giles all agree that Morgan is their one suspect, a camera and a cameraman can be seen in the reflection of the glass in the door labeled "Librarian", right after Buffy says, "Uh, priority check, Giles." A boom mic is also reflected in the top right of the glass when Buffy finishes her line with "talent show murder!" The mic can be seen throughout the rest of the scene from two different camera angles.
Revealing mistake: When Sid stabs Marc after he has transformed, there is no blood on the knife that he uses afterwards.
Other mistake: When Sid gets into Buffy's bed and Buffy screams, her mum immediately comes into her room to find out what's wrong. Now, unless she sleeps on the landing, Buffy's mum is really quick at getting out of bed.
Visible crew/equipment: When Buffy is searching through the backstage area (before the chandelier falls on her), there is a mirror in the background. At one point in the mirror, you can see the reflection of a crew member. Pause for a better look but it can be seen at normal speed.
Revealing mistake: When Buffy breaks into the locker, the locker is already damaged. Probably from a previous take.
Suggested correction: Watching the scene, the only real damage is a slight dent that isn't all that close to the lock (she basically punches out the lock to open it). And to be perfectly honest, a dented locker isn't out of the ordinary for a high school. I saw plenty of dented, bent and damaged locker doors all the time back in high school. It's entirely probable that it is damaged from a previous take, but I think it's also within that sort-of realm of possibility (since like I said, dented lockers aren't out of the ordinary) that it might not be considered a mistake. I'll leave it up to people's votes.
Continuity mistake: When Giles walks up to Buffy, Xander and Willow after The Body is found at the beginning of the episodes, watch the extras walking by in the background. They don't always match as the shot cuts back and forth between Giles and Buffy/Xander/Willow. Ex. When Buffy asks "Vampire?" nobody is walking behind her, but when it cuts to the opposite angle, suddenly two students are walking by from Buffy's direction that weren't there before. This sort of thing happens at least two or three times.
Continuity mistake: After the credits, when Buffy, Xander and Willow sit down to talk about what to do for the talent show, two people walk into the background behind Buffy and are standing there talking throughout the scene. At one point, it cuts to Xander for about one second when he says the line "Whatever happened to corporal punishment?", then cuts back to Buffy, and the two people behind her are suddenly gone. There's not enough time for them to have walked away between shots - they simply vanish.
Other mistake: In the very first shot (a long tracking shot backstage), watch the girl practising with her tuba. At one point she pulls her mouth away and then leans back in and starts playing some notes. But you can tell that she isn't blowing into it, and in fact, her mouth isn't even quite touching it. So it shouldn't be producing any notes. Then, almost immediately after, you can also see the actress purposely moving the tuba (sort-of rotating it to her right) to get out of the cameraman's way.
Continuity mistake: When Snyder grabs Buffy after she breaks into the locker, when she turns around, he is holding her hand up by the wrist. The height he is holding her hand up changes a few times between shots. Sometimes it's up by her face, other times it's level with her neck, at one point it's down by her shoulder, etc. Obviously snippets of a few different takes were used, and he wasn't holding it up at exactly the same height each time.
Nightmares - S1-E10
Revealing mistake: The Master's artificial thumbnail is visibly falling off in a few shots of the opening teaser. (00:01:30)
Nightmares - S1-E10
Continuity mistake: In the season 1 episode "Nightmares", Buffy is talking to Willow about her mom's divorce and she shuts her locker. A few seconds later she shuts her locker again. (00:02:55)
Nightmares - S1-E10
Revealing mistake: When Xander punches out the evil clown if you freeze-frame you can see the punch missed the clown by quite a distance, I noticed it without freezing it but it's better to see if you do. (00:35:35)
Nightmares - S1-E10
Continuity mistake: In the scene where we see Buffy's mum drop her off at school, when Buffy is worrying that her dad will not show up, we firstly see Joyce's seatbelt is on. We then see her again and it is still on. However after the shot goes back from Buffy to herself, we can see her seatbelt is off. Then when the shot comes back to her again it is back on.
Nightmares - S1-E10
Revealing mistake: When Willow opens the door to the basement to look for Buffy, the shot looking up the basement's staircase shows Willow's shadow on the inside of the door. In the previous shot when she opened the door, it opens towards a wall and a staircase to the second floor. But there is no light source coming from the wall or the staircase, giving the impression that the basement was filmed on a separate set.
