In the bedroom: Her
vagina is the "peanut
butter" whereas her anus is the "chocolate". It is fine to get some peanut butter in her chocolate but not the other way around. "Chocolate in the peanut butter" is this unfortunate state.
Usage outside of the bedroom:
Computer programmers may exclaim this when they mix up the binding of their
duck-typed variables with incompatible variables for an unfortunate result.
"Oh no, I got chocolate in the peanut
butter."
(
said in
front of co-workers in which whom only have experienced the bedroom version) "Oops, did I just say that?"