Although there are many individual activities that need to take place in order for a sacrifice to be successfully brought in the Temple, there are four specific acts during which inappropriate thoughts can make the korban invalid. They are:
1. Shehitat ha-korban – at the time the animal is slaughtered
2. Kabalat ha-dam – when the blood is collected
3. Holakha la-mizbe’ach – when it is carried to the altar
4. Zerikat ha-dam – sprinkling the blood on the altar
What are considered “inappropriate thoughts”? There are three types of thoughts, or, indeed, spoken words (according to most opinions) that can make the sacrifice invalid.
The most severe of these is when, in the midst of one of the activities mentioned above, the kohen thinks that he will sprinkle the blood or sacrifice the meat of the korban at the wrong time. This type of thought will make the sacrifice pigul (“a vile thing” – see Vayikra 19:7); the person who brought the sacrifice will need to replace it with another, and anyone eating from the meat of the korban will be punished with karet (excision).
Another possible problem would occur if the kohen thinks that he will sprinkle the blood or sacrifice the meat of the korban in the wrong place. Under those circumstances, although a replacement korban would need to be brought, it is not considered pigul, and there is no penalty of karet for someone who ate the meat of that sacrifice. These two cases apply to all korbanot.
There is a third case where a thought will invalidate a sacrifice, which applies only to a korban hatat (a sin-offering) or a korban Pesah (the Passover sacrifice). If the kohen does not think that it is for this particular type of sacrifice, and mistakenly believes that it is for a different one, the korban hatat or korban Pesah will be invalid. In the case of the korban Pesah, even thinking that it will be used for a mundane purpose will ruin it.