I'm allergic to pretty much every single variety of nut in existence, but I'm fine with that since nuts taste terrible anyway.
This isn't really a dietary restriction so much as personal preference, but I refuse to eat colored frosting. Red, blue, and green are the worst. Yellow is slightly less bad, but still pretty bad. They just make me sick when I eat them. I suspect it's probably something in the dye that makes it taste so bad given regular white frosting and chocolate frosting taste just fine to me.