In the hospital, the mental patients each start mumbling the phrase "What's the frequency?" This is an obvious reference to the infamous event, in 1986, where CBS Newsman Dan Rather was accosted by (then) unknown assailants. One of the assailants, apparently deranged, kept repeating the question "Kenneth, what is the frequency?" (which meant absolutely nothing to Dan Rather)
Spike's intended gift for Buffy is the same box of chocolates with which he practiced his lines on (and bludgeoned) the Buffy mannequin in Triangle (2001).
In her fight with Glory, while Buffy is unable to defeat the Hellgod, she puts up a much better fight, lasting a bit longer and even landing a few good blows, unlike the last time they fought (No Place Like Home (2000)). This occurs in their later fights until Buffy gains the upper hand in The Gift (2001). This shows Buffy's adaptability to fighting stronger opponents, something she shows in her later fights with Caleb in season 7.
This foreshadows Dawn's penchant for kleptomania, which will be focused on in the next season.
The 12th or 13th episode of each season is traditionally when Buffy celebrates her birthday; her birthday takes place in episode 13 of season 2 (Surprise (1998)), episode 12 of season 3 (Helpless (1999)), episode 12 of season 4 (A New Man (2000)), and episode 14 of season 6 (Older and Far Away (2002)).