*(Lomdus alert: this question is not for beginners.)*

In [Maseches Demai (5:10-11)][1] 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.  Either way: if the demai was not tithed, then they are both untithed, so the teruma should be good.  And if the demai was tithed, then it turns out he just took extra-large teruma from the vadai.  What's the problem, that the mishna requires him to take teruma again?


  [1]: http://kodesh.snunit.k12.il/b/h/h13.htm