Chocolate in the peanut butter 

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?"

Chocolate in the peanut butter by logicmoo January 16, 2018