2,643. Eating Something Tiny

Hilchos Shevuos 3:11

Let’s say that someone took an oath not to eat a certain loaf of bread and he later suffered pain because of it. If he eats the loaf thinking that doing so is permitted because of his pain, he is considered to have violated his oath unintentionally. He is exempt from bringing an offering because he hasn’t changed his mind due to new information; rather, he knew it was prohibited and ate it by mistake.

Hilchos Shevuos 4:1

If a person takes an oath not eat to anything that day and he then eats smaller than an olive-sized portion, he is exempt. This is because eating doesn’t mean a volume smaller than an olive; it’s the same as if one ate half the minimum measure for a neveila or a treifa, etc. If he took an oath not to eat a particular thing and then he did so, he is liable even if the thing about which he took the oath is a single mustard seed or smaller.