Even when you are trying to do the right thing recycling wise.
Turns out that some legacy materials, designed initially for one thing, then used in newer things for some time now, and subsequently being recycled into new products, because we want to try harder to be responsible for the whole lifecycle of an item of course; that because of these legacy materials, you can have recycle come back to bite you in the ass.
Case in point, black plastic. Something that started with older electronics, inherited their need to not burn too easily, but now get recycled into new products where the "not so burnable" property comes with a chemical component you don't want to be part of any food container, or child's toy, for that matter. But how would an ecycler know unless informed of this new concern. And maybe a new way should be found to insure electronic devices don't burn so easily as well.