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.