My scenario runs like this: the one-shot was a prophetic Slayer dream, while in reality The Twilight/Caleb (to crush Buffy completely) reveals to her that Spike is alive and didn't bother even to call her. "He told you he will always love you - but he doesn't want you anymore! Mwahaha!"
Buffy: "Oh, it's a betrayal! The closest, the most unexpected..."
Fans: "Joss, it's cheating!"
Joss: *smirks*

I'm not sure I'm right, I'm just 100% sure I'm cynical.