I've heard that you cannot put royal icing on butter cream b/c the crisco breaks down the royal icing.....anyone know if it's true? I would like to make a butter cream iced cake with royal icing snowflakes

I have seen alot of cakes with bc and royal accents. I am making a bc cake with royal snowflakes in January. You should be fine!

Once royal is properly made it *CAN* be used on b'cream.
It does depend on what deco is royal and the recipe for b'cream used but for the most part there is no problem. This is an 'old wives' tale that refuses to die

Just don't put royal icing into the fridge! The humidity in there will start breaking it down.

great thanks for responding....i'm going to make a snowflake cake

I honestly have never heard of this. I make royal icing flowers all the time and put them on buttercream cakes. Never had a problem. Never had a problem in the fridge either, even sitting overnight. (gum paste will get soft on my in the fridge though) I agree, must be an old wives tale...