When Spike faces his opponent in the test, he remarks, "Here we are now, entertain us," which is a quote from the Nirvana song "Smells Like Teen Spirit".
Anya refers to Andrew as "what's-his-face, the other guy." This is part of the running joke of the Scooby Gang being unable to recall Andrew's name, also present in Gone (2002), Never Leave Me (2002), Grave (2002) and the comics Turbulence, and Own It, Part Five.
Nicholas Brendon does the opening narration "This is what happened this year" which precedes a longer-than-usual recap of all the major events of season six.
Buffy states that killing someone changes you, followed by "Believe me, I know." She has twice thought she had killed a person: the robotic Ted in Ted (1997) and, more recently, when she was framed for the death of Katrina in Dead Things (2002). This also reflects her experience with Faith when the Slayer went rogue after accidentally killing Allan Finch in Bad Girls (1999).
Buffy's warning to Willow about not wanting to hurt her, only to be attacked in response and then attacking back, claiming she didn't want to but is still willing is similar to a situation in Becoming: Part 2 (1998) between Spike and Drusilla where in mid-fight, Spike says "I don't wanna hurt you, baby" before being forced back at his throat where he then easily manages to release himself from her grasp and punches her, then saying "Doesn't mean I won't".