Rewatching with the GF who is a first time watcher... s6e6
As the title says, I am a LONG TIME watcher, and I am introducing my gf to the show.
In s6e6, when Anya and Dawn are doing their Money Dance, Xander says, "I'm gonna marry that girl."
And Buffy says, "What? She's 15 and my sister, so don't eve- Oh."
And after knowing what I know of the comics, holy crap foreshadowing!