Well some basic rules are followed--(or not)-I am thinking of the Friend's (TV) episode where one of the characters was making a "trifle" and two pages of the cook book stuck together and she had mixed up a trifle recipe with one for shepard pie--and had sweet custard, jam, ground lamb and mashed potatoes (or some other strange concoctions) in the dish.

while it it not uncommon to mix sweet with savory (mint jelly and lamb) there are some limits... even for creative cooks. I have read about some brownies that include "hot chilies" but i tend to like my desserts not to be mouth sereing! and while maybe a case could be made for hot chilies in brownies-- you wouldn't have both hot chilies and meat-- and still consider it a dessert--

there is always my favorite Monty python-- about the "interesting" chocolates-- like the Crunchy frog chocolates..
--look out, hyla-- it where you take a sweet young frog, boil them in the finest spring water, and cover them in chocoate.. the health inspector asks if the frog have been gutted or de boned-- but not they haven't-- all natural-- and the bones are what make them crunchy-- I am sure some one will google the monty python site and get use a link to the complete skit.. some of the chocolates did not follow "the rules' for food!