Shevuot 2A

מתני׳ שבועות ⁠– שתים שהן ארבע.

Mishnah: With regard to oaths on an utterance of the lips, there are two types that are actually 4 types. The Torah specifies only two types of oaths whose violation renders one liable to bring a sliding-scale offering to atone for his transgression (see Leviticus 5:4): Where a person takes an oath to perform some action, and where he takes an oath to refrain from performing some action. With regard to both types, the Torah explicitly mentions liability only for an oath pertaining to one’s future behavior. Nevertheless, the rabbis derive that one is also liable for a violation of both types of oaths when they pertain to one’s past behavior. Accordingly, although only two types are explicitly mentioned in the Torah, the rabbis derive that there are actually 4 types.

ידיעות הטומאה ⁠– שתים שהן ארבע.

The Mishnah lists similar groups of halakhot. With regard to cases of awareness of the defiling of the Temple by entering it while one is ritually impure, or defiling its sacrificial foods by partaking of them while one is ritually impure, there are two types that are actually 4. It is prohibited for an impure person to enter the Temple (see Numbers 19:20) or to partake of its sacrificial foods (see Leviticus 7:19–20). If one transgressed either prohibition during a lapse of awareness, then upon becoming aware of his transgression, he is liable to bring a sliding-scale offering (see Leviticus 5:2). The Torah specifies that one is liable to bring the offering only in the case in which he had a lapse of awareness of the fact that he was impure. The rabbis derive that one is liable not only in these two cases, but also where he was aware of his personal status but had a lapse of awareness concerning the identity of the place he was entering or the status of the foods he ate.