At the very beginning, when Giles and the Scoobies are walking in the graveyard, there's a gravestone clearly marked "Snyder", a reference to Principal Snyder, played by Armin Shimerman, who was a major character in the high school years.
When Anya is trying to clean Buffy's shirt she says, "It must be blood", but then continues, "Or pizza". In season seven's Conversations with Dead People (2002), Dawn is eating pizza and drops some on one of Buffy's shirts, and says, "She will think it is blood".
The clip of the first vampire stepping into the alley was footage recycled from the teaser of The Gift (2001).
Wood tells Buffy that he went through a whole "avenging son" phase in his twenties. As he was born sometime after his mother's Tento di Cruciamentum in 1973 and was 4 years old in July 1977, he's not over 30 at the time this episode takes place.
Xander originally asked Lissa a very logical question: "Did you hatch out of a praying mantis egg in the old high school?"