Standard disclaimer: I'll often speak of foreshadowing, but that doesn't mean I'm at all committing to the idea that there was some fixed design from the word go -- it's a short hand for talking about the resonances that end up in the text as it unspools.
Standard spoiler warning: The notes are written for folks who have seen all of BtVS and AtS. I'll be spoiling through the comics as well. Basically -- if you are a spoiler-phobe and haven't seen or read it all, read further at your own risk.
Standard Credits: I've written the material in black; Strudel (aka my Bro) writes in blue; local_max writes in purple. Or at least, that's what they've done when I finish editing and formatting!
Buffy 1.11 Out of Mind, Out of Sight, in Which an Invisible Girl Helps Us See Many Things