(Lomdus/Jargon alert: this question is not for beginners.)
In Maseches Demai (5:10-11) we learn that taking teruma from a vadai (=untithed produce) on demai (=possibly untithed produce) does not work, even on the vadai itself.
I totally don't understand why this should be. Mimah nafshach Either way: if the demai was not tithed, then they are both teveluntithed, so the teruma should be good. And if the demai was tithed, then it turns out he just took extra large-large teruma from the vadai. What's the problem, that the mishna requires him to take teruma again?