Related על נזיר 72:1

He had before him two containers -- one of terumah and one of chullin (unconsecrated produce), and in front of them (i.e., the containers) they had before them two seahs [of produce], one of terumah and one of chullin (Yevamot 82a:11). And [produce] fell from both seahs inside of both [containers], and it is unknown if this [produce fell] inside that [container], or if that [produce fell] inside this [container]. [He may therefore make the following declaration:] "Behold, I say, the terumah [produce] fell inside the terumah [container], and the chullin inside the chullin." If it was untithed produce, first tithe, or second tithe, behold, it is forbidden [to make such a declaration], as we do not permit [a designation of status where there is] a doubt as to whether it is a forbidden mixture, except [MSS add "דבר" = in a matter] where there are permissible factors (i.e., circumstances that may yield everything permissible). [Note: In the first case, since it is possible that the terumah produce landed in the terumah container, and the chullin in the chullin, the Sages permitted the designation, since both containers would be permitted. However, in the second case, if there was one container of chullin and another of untithed produce (or first/second tithe), and the seahs of terumah and chullin got mixed up, at least one of the containers would necessarily contain an impermissible mixture -- either terumah mixed with chullin or terumah mixed with untithed produce (or with first/second tithe) -- and since no retroactive designation could render both containers permitted, no declaration is allowed. See Minchat Yitzchak here.]