BHA/BHT/ethoxyquin,artificial dyes and flavors,any foods that are not listed as safe or unsafe along with those which are listed as unsafe,or otherwise questionable ingredients.Things like digest,derivatives,byproducts,unspecified meat or fat are also not safe and in the case of digest and derivatives are the same as byproducts and is an easy way for manufacturers to get out of directly stating that they essentially contain slaughterhouse waste and other nasty ingredients
In the UK,you'll see things like unspecified antioxidants.This is not safe either,and should be instantly considered unsafe as it would be completely fine for these products to have BHA/BHT/ethoxyquin,which are known to cause cancer and are completely unsafe.I've also seen plasma and bloodmeal,which are both unsafe for any animal.