One of the sages testified that he had seen Rabbi Judah the Nasi (editor of the Mishnah) burping while praying, placing his hand over his mouth to yawn, sneezing, spitting, and taking bothersome lice out of his clothing while he was praying. On the other hand, if his talit fell to the ground while he was praying he would not pick it up so as not to be distracted from his prayers.
The scholars asked: How did Rabbi do these things during his prayers? They are completely contradictory to Halacha, which states that one who burps or yawns during prayers is rude. One who sneezes during prayers is committing an ugly act. One who spits during prayers is as one who spits before the king! Rabbi’s actions are inappropriate and no one should do them during prayers.
The scholars reconciled the contradiction between Rabbi’s actions and Halacha: when Rabbi burped and yawned during prayers, it was unconsciously, and Halacha forbids conscious, willful burping and yawning. Rabbi Judah the Nasi sneezed, but the “sneeze” which was forbidden by Halacha was the expulsion of gases from the lower body — flatulence. When Rabbi spit during prayer he did not spit directly on the ground, but into a cloth, and this manner of spitting is permitted by Halacha. Thus they reconciled Rabbi’s actions with Halacha.
The scholars also related that when a different sage, Rav Ashi, had spit in his mouth during prayers, he would spit on the ground behind him and not into a cloth, because he was fastidious.
(Babylonian Talmud, Tractate Berachot 24a-b)