2,651. Conditioning an Oath

Hilchos Shevuos 4:16

Let’s say that someone takes an oath not to eat a large loaf if he eats a small loaf. If he forgets his condition when eating the small loaf and he later eats the large loaf intentionally, he’s liable.

Hilchos Shevuos 4:17

Let’s say that he remembered his condition when eating the small loaf and he knew that eating it would render the large loaf prohibited. He then forgot and ate the large loaf, thinking that it wasn’t yet prohibited. In such a case, he’s exempt. If he ate both loaves unintentionally, he’s exempt, and if he did so intentionally, he’s liable regardless of whether he ate the large loaf first or second.