The plot of the episode, involving an alternate reality where the same people are present, but their personalities are different is drawn from Star Trek's "Mirror, Mirror."
The Master fulfills the prophecy again by killing Buffy, just like in "Prophecy Girl"
When Cordelia meets her old gang, Harmony says she found someone better fitted for Cordelia than Xander was and shows her Jonathan. Cordelia toyed with dating Jonathan in "Reptile Boy"
One of the things Cordelia asks herself is why she has been bitten by snakes. She was bitten by a snake in "I only have eyes for you"
When Xander throws a match at Angel, Willow remarks that the match almost lit her hair. In Dead Man on Campus, Alyson Hannigan's character's hair did get set on fire.
Evil Willow is sucked out of the previous episode, to do her thang, only to return, and die, again.