Pesachim 59 - What did you have in mind?

In bringing an offering in the Temple, there were four steps: slaughtering the animal, receiving its blood in a vessel, carrying the blood in the vessel to the Altar, and throwing the blood on the Altar. If the sacrifice owner was knowledgeable in slaughter laws, he could do it himself, but the remaining three steps had to be done by a kohen priest.

For each step, one needed to have the proper intention. If he is bringing a Passover offering, he should have in mind that he is slaughtering the animal as a Passover offering, receiving the blood for a Passover offering, and so on. If he says, "This is a Passover offering, a peace offering," the offering becomes invalid, and its service should be discontinued. This means that to have the Seder tonight, he will need to bring another animal sacrifice.

Thus, one can disqualify an offering with a wrong statement. Some say that even a wrong thought can ruin an offering. As a practical matter, the kohanim priests were instructed to be silent and have in mind that they are bringing the sacrifice for the right purposes without specifying what it was.

Art: Agnus Dei, by Francisco de Zurbarán