Terumot, Chapter Three, Mishnah Eight
This mishnah deals with a person who intends to say one thing but instead says another. Is there any validity to either his words or to his intent?
1) He who intends saying terumah and says tithe, or tithe and says terumah;
2) Burnt-offering and he says peace-offering, or peace-offering and he says burnt-offering;
3) [I vow] that I will not enter this house, and says that house,
4) That I will not derive any benefit from this [man], and says from that [man], he has said nothing until his heart and mind are at one.
This mishnah seems to be quite understandable, so I will keep it brief.
In order for the persons statement to have any validity, he must actually say what he intends to say. Therefore, if he intended to say that something would be terumah and he instead said that it would be tithe, that which he separated is neither tithe nor terumah. The same is true for all of the other examples in the mishnah.