Me'ila 16 - How Prohibited Foods Combine
All five foods called "terumah" - "first portion" combine to create a prohibition. That is, if one ate small amounts of them, but together it was an olive volume, he is liable. The foods are terumah of the Kohen, terumah that the Levi gives to a Kohen, doubtful terumah (also called "demai", "what is it?"), terumah of the dough (called "challah"), and first fruit - the last two, because they have an additional name of "terumah."
All of the " nevelah " meats (animals that were not properly slaughtered) combine with one another, and so do all creeping creatures. In commenting on that, Rav said that "nevelah" meat of kosher and non-kosher animals do not combine for the prohibition of eating, because they are prohibited by different phrases in the Torah, and because one prohibition cannot be superimposed on the previous one. In our cases, a non-kosher animal is already prohibited, and it cannot become more prohibited as "nevelah" - not slaughtered.
However, Levi said that they do combine anyway. In principle, Levi agrees that prohibition don't get superimposed, but nevelah is an exception, because of an extra prohibition for it in the Torah.
Art: Jacob Foppens Van S - Still Life With Oysters