Other mistake: Willow's computer beeps twice when her first email from Malcolm is received through before she is kidnapped by Fritz, but only beeps once when the second email is received.
Seeing Red - S6-E19
Other mistake: At the Bronze, when Warren punches Xander the first time, Warren never actually punches him, and Xander starts to go up in the air before you even hear the sound of the punch.
Other mistake: When Buffy and Angel escape from Spike, Dru and the Judge they jump down into the sewer, then run all of five metres before going up the ladder to escape. When they come out of the sewer they are in a very open area with the factory nowhere in sight. They have not travelled far enough for this to be the case. (00:39:15)
Other mistake: The newspaper that Xander is reading in the library has the same text over and over again and does not at all match the headlines. (00:18:05)
Other mistake: 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.
Other mistake: In "Doppelgangland", Vampire Willow struggles to remember Cordelia's name. However, it is still Willow, she knew Cordelia from being human (we see this in The Wish as well).
Other mistake: When the detective is interviewing the woman in the alley he says "So you heard the man scream about what time?", but Finch didn't scream as he died.
Other mistake: We're shown a variety of photographs and pictures of different members of the gang at one point. One of those, the one of Xander and Anya, is actually a shot from the episode "Something Blue". I doubt very much that during the events in that episode, someone conveniently decided to take a photograph.
I Was Made To Love You - S5-E15
Other mistake: According to Buffy's watch, the first scene with her, Dawn and Joyce starts at 4:23 PM and ends at 4:24 PM. But the scene lasts much longer than that.
Lies My Parents Told Me - S7-E17
Other mistake: Robin uses the song that Spike's mother sang to him to provoke him into attacking him. The song is played on his computer through Apple's media program "iTunes". He clicks his computer's mouse to start the song, but when we see the monitor the cursor isn't placed on the Play button. Robin doesn't jostle the mouse when he presses it, it remains completely still on the desk. And the only way to play a song through iTunes using a single mouse-click is by clicking on the Play button.
Other mistake: In episode "Helpless" in season 3, after the council member bitten by the crazy vampire rises, his bite marks heal. But in season 7's "Lies My Parents Told Me," it shows Spike's mother already risen and her bite marks are still there.
Other mistake: At the end of the episode Willow mentions going to the Bronze on a Wednesday night, Buffy also mentions to Giles that she got almost everyone killed on the second day of school. From the different clothing being worn each day, it's obviously been more than 2 school days and should instead be Thursday by the end of the episode.
Other mistake: When Robin Wood is burying Jonathan, Jonathan's face flinches when the first heap of dirt lands on his legs.
Other mistake: When we first see one of the Hellhounds at the clothes store that Cordelia works at, it jumps through the window, smashing the glass, and then attacks a customer who's wearing a tuxedo. But the Hellhound attacks the customer several seconds after it smashes through the window, giving the customer a chance to escape. But the customer just stands there unaware of the Hellhound, having not heard the window being smashed.
Other mistake: The Master's skeleton, before it is destroyed, looks to be female and not male. (Note the wide hips and large birth canal-like hole.) Also in the Next season's "When She Was Bad," they have another shot of the same skeleton, however the hips are much smaller and more male. (00:43:55)
Other mistake: At the end the library that was rigged with explosives blows up. But the school's main entrance and the clock tower blow up as well, even though they are not rigged with explosives.
Other mistake: In the basement when Death throws Ryan to the floor, Ryan lands pretty softly considering that he was just thrown pretty hard.
Other mistake: In the scene where Spike escapes from a blind Giles, he pushes Giles out of the way and does not get a shock to the head from the chip in his head, which normally goes off with the most minor hint of violence.
Other mistake: After Gage turns into a monster and fights Buffy, the two monsters slide into an open grate. During the shot, there's some sort of weird "glitch" in the background where the garbage can and wall seem to slightly move. All I can assume is that a few frames were trimmed to speed up the shot and it may have exaggerated subtle camera movement, making the background appear to "jump" slightly.
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.