Once Buffy beats Spike, he runs off. When they show Buffy again, you can see him stand and walk around the corner.
In the opening scene when Buffy is fighting the vampire, the camera pans towards Spike's perspective while secretly filming the event. The vampire Buffy fights is visible on the recording screen even though its been well established in the series that vampires don't have an image to be reflected or seen.
As the Scoobies are being chased by Spike, Xander states "we need a triage". A triage is a medical term used to assess the degree of urgency in wounded or ill patients. Xander should have said "we need a blockhouse", which is a small fortification allowing for defensive fire in all directions.
During the opening fight sequence when Buffy stabs the vampire with the sign post, she misses his heart by a noticeable margin. It has previously been stated that the stake needs to pierce the heart to attain the kill. Thus, the vampire should only have been wounded and Buffy would have needed to stab him again.
A vampire enters the Summers home after Angel by simply walking in the house, yet vampires must be invited into a home before they can enter. The "vampire" in this case is not an actual vampire, however, but merely another high school student who has transformed into the monster he was dressed as for Halloween. We see him as a human earlier, escorting a group of children while trick-or-treating. Presumably, the laws which prevent real vampires from entering a house uninvited do not apply in this case.
Willow is a ghost. However, when Giles tells her to leave the shop, not only did the curtain move when she leaves, you can also hear her footsteps and hear her open and close the door behind her. Take Willow out of the frame and you have the sound of footsteps, a moving curtain and a door opening and closing. These are all facets of ghost stories and films. Phenomena that all ghosts can do that arouse fear in the mortals in the room. In this case, she is a ghost that is currently using her ectoplasm to be seen.
When Willow and Buffy are reading the Watchers Diary, Willow asks the name of the girl who is pictured. Buffy says it doesn't say. In close-up of the page, we can clearly see the name "Sarah Greenwood" next to the picture.
Vampires cast no reflection. Since video cameras use mirrors as part of their focusing method, vampires would not show up on videotape.
Willow remarks that in 1775 "Angel was 18, and still human." It is later established that Liam was born in 1727 and became the vampire known as Angel in 1753.
The role of the Roman god Janus is somewhat distorted here. He was the god of transitions; his name is related to 'janitor' (which originally meant a doorman) and 'January' (the gate between years). His two faces look to the past and the future. A more appropriate deity would have been Eris, the goddess of chaos, of which Ethan calls himself the "faithful, degenerate son." Janus was likely used because of his depiction as having two faces, an idea which fits clearly with the theme.