In "The Witch", when Willow fills the test tube with the yellow liquid in science class the test tube is filled half way but in the next shot, as she hands the test tube to Buffy, the test tube is filled only a quarter of the way.
In "Teacher's Pet" Miss French is making herself a sandwich her sleeves are rolled up to her elbows but in the next shot, the close-up as she puts the bugs on her sandwich, her sleeves are down to her wrists and in the following shot her sleeves return to being rolled up at her elbows.
In "I Robot, You Jane", the guy working for the Moloch demon opens Buffy's school records, and under 'Status', it says that Buffy is a senior at the high school. That's wrong. In the first season, Buffy is a sophomore.
In "The Puppet Show" when Xander made Sid go 'redrum, redrum', it wasn't in the script, Nicky ad-libed it. When Willow ran off the stage during the dramatic performance at the end of "The Puppet Show", it wasn't in the script. Later, in 'Nightmares' we find out one of her biggest fears was being on stage.
After Buffy is killed by the Master in "Prophecy Girl", she falls in the pool of water and her hair is up and her arms are under her. But when Angel and Xander find her her hair is down and her arms are by her sides. Amazing considering she is unconcious throughout.

Season Two

In "When She Was Bad", the ritual to bring the master back to life involves the masters bones. But how could that be if vampires turn to dust when they die?
In "School Hard" when Buffy is dancing with Willow and Xander, Spike says that he is going to call the police because there is someone outside trying to bite a girl's neck and Buffy runs outside, Spike has no jacket on. Then when he is outside, he has one on.
In "Halloween" when Giles is talking to Ethan and Giles tells Willow to go, you can hear the door clearly shut, but Willow has been turned into a ghost, so she can't touch anything.
In "The Dark Age," Buffy's microphone is clearly showing. In "The Dark Ages" when Giles is calling everyone involved in the Eyghon thing, he crosses out the girl's name and puts his glasses down near the notebook. then the camera zooms in on the list of names and the glasses are on top of the notebook.
In "What's my Line part 1", when Buffy and Kendra are fighting, Kendra throws Buffy into a table and the table breaks.But in the next shot, the pieces table are gone.Where did they go?
During "Suprise" you see Angel in pain in bed - Nude, then all of the sudden he has underwear on, then he mysteriously runs outside FULLY clothed and falls down in the rain and is SOAKING wet...then when he bites the woman's neck...he's mysteriously "dry".
In "Innocence", first Buffy has no nail polish on, then she gets up and leaves Angel's bed and suddenly she has nail polish on.In "Innocence", when Angel comes into Jenny's Uncle's room, he just walks in...Doesn't a vampire have to be invited?? In "Innocence", when they are all in the mall, there is a shot of Drusilla sitting on the floor where she has quite a definite reflection in the shiny pillar thing behind her. I thought vampires didn't cast a reflection....?
In "Phases", Willow runs into the library right after running away from Oz the werewolf. When she comes in Giles isn't wearing any glasses, next shot glasses, next shot no glasses, then again glasses. Also in "Phases", while Willow was running away from Oz the werewolf she falls then the hunter guys come and she runs off with dirt on her overalls. But when she leaves the library with Buffy and Giles, there isn't one speck of dirt on her. Giles' car had some trouble starting in the episode "Phases", for the scene in which Buffy and Giles approach the Bronze. So, the crew recruited a couple extras to help push the car as if it were being driven towards the camera. If you watch carefully, you can see the tops of their heads through the car's rear window.

Season Three

In "Anne" when Buffy climbs up the chain, she is wearing Ked-like sneakers, then when she lifts the gate and leaves she is wearing Nike like sneakers.
When Faith asks Buffy in "Faith, Hope and Trick" what her toughest kill was, she remembers killing Angel, then tries to answer by saying, "Do you guys remember the Three?" I realize she's trying to come up with something to avoid telling the truth, but this is an extremely poor choice. She didn't kill the Three! They offered their own lives to the Master as penance for failing to kill Buffy, and Darla performed the honors.
When the vampire versions of Willow and Xander in "The Wish" have finished sucking Cordelia's blood they just throw her dead body aside but when it hits the floor it makes no sound!
In "Consquences", after the box falls on Buffy she becomes stuck and tries to free herself at this point she is lying down on her back and Faith is being attacked by a group of vampires. In the next shot however you see Buffy sitting up in the backround watching Faith fight. The next shot shows Buffy lying on the ground trying to free herself again.
In "Earshot", when Buffy sees Jonathan in the clock tower, the clock says something like 12:40. When she climbs up and jumps in the tower window, the clock says something like 12:15. So what time is it again?
At "The Prom" in the scene where Wesley and Giles are sampling chips, Wesley is holding a red napkin, but when he ogles at Cordelia, we get a new shot of him and his napkin is now yellow.

Season Four

In "The Initiative", Spike tells Willow that he is only 126 years old in the scene where he tries to bite her. But when he arrived in Sunnydale during season two, Giles had told the gang that he was barely 200 years old. Talk about lying about your age.
During the Indian attack in "Pangs", Spike is shot several times by arrows, but when they all sit down to dinner afterwards, there are no holes in his shirt.
During "A New Man", Giles is sitting alone and Willow comes over and offers him a bit of cake which he accepts. In the next shot, Buffy and Riley come over and Giles has no cake so Buffy sends Riley to get a piece. Where did the first piece go because there wasn't enough time to eat it.
In "Goodbye Iowa", Buffy finds Riley's hand is bleeding and she takes off her headband to press the wound with it. While she and Riley are talking, there are switch-off shots between them, and in one one shot, Buffy's headband is back on, then it's off.
In "This Years Girl", at the scene where we see the gang walking through the forrest and they're about to see the demon with his stomach ripped open we can see the sound man in the background holding the sound mike and trying to move out of view.

Season Five

In "Real Me", during the scene where Willow and Tara are in their dorm room and talking about Dawn, the boom mike is seen just poking in from the top of the screen.
In "I Was Made to Love You", when we see Glory transforming into Ben, look at the bottom of the screen and she appears to be wearing a gold strapless dress. But when she has turned into Ben, Ben looks at himself and says 'Oh god' and he's wearing a red strapless dress.
In "The Weight of the World", Willow goes inside Buffy's mind to snap her out of her catatonic state. While there, she sees Buffy as a little girl with blond hair. However, in the episode "Killed By Death" in season 2, we see Buffy as a little girl, and she has dark brown hair. So, what is her real hair color?

Season Six

At the beginning of "Afterlife", Buffy's knuckles are bloody because she fought her way out of her coffin. But at the end of the episode, when we see her hugging Dawn, her hands are perfectly clean and healed.
In "Tabula Rasa", Spike is falls under Willow's spell, and passes out on the magic shop counter. But in the next cut, he's lying straight across the counter, arms under his head, as if he simply fell asleep there. How could he do that if he suddenly just passed out?
When Buffy decides to play a cruel prank on that social services lady in "Gone" she starts typing on her computer saying 'All work and no play make Doris a dull girl'. But if you look closely, Buffy never once hits the 'A' key.