A pre-twentieth-century brewer had more options to create frosty suds for his customers. Anything was game for the boil kettle. Every source of starch ended up in the kettle. This wasn't always for the best. During the Napoleonic Wars, British brewers lowered the alcohol on their porters to avoid excess taxes. To keep the desired “kick” they reportedly added agents like tar, lead, and sulfuric acid. Of course, this is rightfully outlawed these days.
An old brewer's maxim says, “If it has starch, it can mash.” Whether it's an odd form of sugar, old corn tortillas, potato flakes, or chocolate cake, there's little that can't ferment. To preserve head retention you must de-fat unusual brew additions. For example, the chocolate birthday cake was defrosted and boiled briefly. The butterfat was skimmed from the top of the cool slurry.
Other crazy ideas gleaned from fellow homebrewers include: rye bread, pretzels, apple pie, cucumbers, peanut butter, Jolly Ranchers, Mountain Dew soda, Red Zinger tea, ginger snap cookies, ad infinitum.