Jeanty's cover for Buffy #8
Source
According to Scott Allie, "people are gonna assume this is a prank or a fake cover, but it's not. She really does get her arm torn off by a zompire in the middle of a big battle involving Spike and Dowling. It's not a dream, unlike much of the action in Buffy #5. Why show it on the cover? It gets to be a drag saving all the juiciest stuff for the insides, and we think that it'll still have some surprise impact when it happens."
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In any case, it happens in the April issue, and we already have a May cover for Free Comic Day issue where Buffy is okay again (although she keeps her stake in her left hand).
The only idea about "the juiciest stuff" that comes to mind is that Andrew has built his little private Tardis pre-season 9. That's how he knew in advance what happens to Buffy, so he built an artificial hand for her in issue 3.