AI have two big cakes I been working on for a couple kids whose brother is ill. One of them is a spiderman bust and it went perfectly together no issues. Until it was complete. I moved on to the second cake and when I looked at spiderman, his face had come apart. I had used edible writers to draw where I would pipe the web lines (I didn't trust my shaky hands without a guide.) The fondant separated only in places where Ihad written with edible writer then piped over with black frosting. I have repaired the best I can and all I can do is hope the face stays in tact now until it is cut at 2pm today, but for future reference I'd like to figure out what went wrong. Has anyone had this happen with edible writers combined with the buttercream?

I can't tell you specifically, because I don't know enough about the fondant (how thick was it? what type of writer did you use? what did the separations look like).

But it sounds to me like your fondant might have been too thin, and you might have pressed a little hard with the writer, indenting it a little, and making the fondant too thin to support its weight at that spot. 

FWIW, when I need to follow something on fondant, I take a veining tool, and gently drag it over the surface of the fondant, rather than drawing on it.