This is an old story - how old, I have no clue. Basically, Spike comes back as a human and Buffy, not realizing he's human, nearly kills him. I don't remember much but I do know he goes to live with Angel for a while

Maybe it's Humanitis by Saber Shadowkitten?

http://www.sinister-attraction.net/stories/s/humanitis.htm

no...not it

I think the story you're talking about is "they didn't know". I don't remember who wrote it but I remember I found it on fanfiction.net

can u give a little bit more info?

Dang. I read this long ago but I just can't recall the name or author. It was a long story, that I remember. If anyone has even a part of a name or part of an author name I could probably find it easily. I can't find anything using 'They Didn't Know' and that name doesn't ring any bells either so I'm not sure that is the same story. I could be wrong, but I don't think it was called that.