If it exsists, there is a porn of it, if there isn't there will be, there is NO exception to this rule as of now.
You cannot be 100% specific about this rule.
I saw a rule 34 of Scooby Doo - Correct.
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)