Bread. We covered this. It’s a sponge of doom. Cook your stuffing separately in a baking dish. Get crispy edges. No salmonella.
Rice. Or quinoa. Or any grain. It will swell into a concrete brick. You’ll need a chisel to get it out. Trust me.
Butter. Don't put a stick of butter in the cavity. It just creates greasy, unseasoned fat. Put butter under the skin instead. That’s a whole other article.