Außen knusprig, innen Ge(n)schmack!
It looks crispy and tasty, but in fact it isn't.. It is fool's gold!

Solution

It does play with the German word Geschmack, which means flavor.. The idea behind is, although it looks from outside crispy and tasty, it just a "fool's gold" (der Schein trügt). Just because something is not visible, it does not mean that everything is okay with it! Don't judge a book by its cover!!!