Kingman's Bluff, where Willow tries to invoke Proserpexa, is the very same area where Angel attempted to commit suicide in Amends (1998).
In the DVD commentary, director James A. Contner says that while filming the scenes with Willow and Xander on the cliff, the wind churned up dust, which blew into Alyson Hannigan's eyes. Because of the dust, Hannigan removed her black contacts and her eyes were blackened digitally in post-production.
This is one of only two episodes in the entire series where you can briefly see one of Sarah Michelle Gellar's real life tattoos. It is seen when Giles and Buffy are talking in the training room (as she is bent over the vaulting horse laughing) and later when Buffy is crawling out from the grave with Dawn. It can also be seen in the first act of Lessons (2002).
When Willow sends the flame ball after Jonathan and Andrew, she says, "fly, my pretty, fly." She then turns to Buffy and says, "See what I did there?" It is widely believed that "fly, my pretties, fly" is a quote from The Wizard of Oz (1939) when the Wicked Witch sends her flying monkeys to attack Dorothy. However, the phrase does not appear in the film. Willow's sidebar to Buffy may also be referencing the fact that Andrew sent flying monkey demons to attack the school play.