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?"
When a man covers his penis in peanut butter and then fucks a girl in the ass. She then shits over his dick and eats the combination of shit and peanut butter
Dude yesterday was valentines day andni gave the girl a reeses peanut butter fuck