Additionally it is accepted that the rule itself has limitations and you cannot be too specific on the content of the item in question.
Most commonly used on various message boards for various reasons, from humor to cruelty.
You cannot be 100% specific about this rule.
I saw a rule 34 of Micky Mouse and Optimus Prime - Wrong.
(Please note, since I have suggested the second example, it is now being worked on by Insane Japanese Hentai Anmiation Artists)
Known exceptions are:
Rule 34 itself. You cannot rule 34, rule 34.
Also, abstract ideas may not be rule 34'd, it must be a concrete object or be representable as one.