When Giles, Willow and Xander are standing over Buffy's grave, the date on the gravestone marks her birth year as 1981. In I, Robot... You, Jane (1997), Malcolm looks at a file on Buffy where her birth year is marked as 1980, and in another shot from the same scene, it states her birth year as 1978.
At the beginning of the episode, Willow and Buffy are talking about Buffy's parents divorce. When, Buffy closes her locker. There is a cut away shot and then she closes her locker again.
When Buffy and her mother are in the car outside the school at the beginning talking about Buffy's father, her mother's seatbelt comes and goes. In the closeup shots in which both she and Buffy appear, the seatbelt is visible but in the close-up of Joyce only, the seatbelt is not visible.
Giles loses the ability to read as one of his nightmares. His other is that Buffy dies. If he can't read, and no one said anything, he shouldn't be able to know that they're at Buffy's grave. However, it is clearly suggested that people don't experience their nightmares all the time, let alone simultaneously. For example, Xander doesn't keep losing his clothes and Willow isn't being chased by the angry audience all the time. It is conceivable that Giles regains his reading abilities once his nightmare switches to Buffy being dead.
When Willow opens the basement door to follow Buffy's voice, from the outside, the door opens onto the main hallway. However, looking out from the inside, the door appears to open out into an alcove, with the opposite wall being very close.
As Cordelia is brushing her frizzy hair, the comb audibly snaps and clatters on the floor, however the comb is clearly intact as she begins to panic.
Aldo and Willow were attempting to preform the famous duet from Puccini's Madame Butterfly. Willow was dressed to play the title character Cio-Cio-San and Aldo was playing her new American husband Pinkerton. In the actual duet, Cio-Cio-San does not come in until after Pinkerton's first five lines (where Willow asked if it was her turn). The first prompting look Aldo gave her was factually inaccurate. Since this is Willow's dream and the scene is generated by her mind, she is the one making the error.