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.