The sages disagreed in the case of a person who ate half the volume of an olive, vomited it up, and then re-ingested it. On the one hand, he has enjoyed the ingestion as though he had eaten the full volume (half the volume of an olive and then again half the volume of an olive), but on the other hand there is only half the volume of an olive in his stomach. The Talmud rules that it is the enjoyment which determines and not what is found in the stomach, so the person would indeed be liable to punishment.
Therefore another rule was also instituted: It is known that in order to fulfill the commandment of eating matza on Passover Halacha requires the ingestion of an amount of matza at least equal in volume to an olive. It was ruled that if a person has only a half that amount available to him for Passover, he can manage to fulfill the commandment by eating the matza he has access to, vomiting it up, and then re-ingesting it.
(Babylonian Talmud Tracate Chulin 103b)
The Talmudic text:
Rabbi Asi, citing Rabbi Yochanan, said: If one ate half the volume of an olive of forbidden food, regurgitated it, and ate a different half the volume of an olive, he is liable, because he had the pleasure of swallowing the volume of an olive.
Rabbi Elazar asked Rabbi Asi: If he ate half the volume of an olive of forbidden food, regurgitated it, and ate it again, what is the law? What was Rabbi Elazar unsure about? If he was unsure whether regurgitated food is considered digested (and is no longer forbidden), he should have asked about eating the volume of an olive of regurgitated food! If he was unsure whether the criterion is pleasure of swallowing or benefit of the stomach, he should have known this from Rabbi Asi’s law! Rabbi Elazar had no doubt; Rabbi Asi had forgotten his learning, Rabbi Elazar was helping him to remember. He asked, why do you discuss eating a different half volume of an olive – you should teach about eating the same half volume of an olive again, to teach two laws! We would hear that regurgitated food is not considered digested, and that the criterion is pleasure of swallowing. Rabbi Asi did not respond. Rabbi Elazar said to him, “Sage of the generation! You yourself said in front of Rabbi Yochanan, ‘he enjoyed swallowing half the volume of an olive’.”