Pesachim 12B

אלא בין יממא לליליא לא טעו אינשי. ובדין הוא דניתיב לקמיה טפי, אלא שבחמש חמה במזרח, ובשבע חמה במערב.

However, between day and night people do not err, and therefore there is no concern that they might have erred in this regard. And similarly, by right we should give them even more leeway at the end of the period. However, this too is not a reasonable discrepancy, as at 5 hours of the day the sun is in the east and at 7 hours the sun is in the west. No one confuses the morning with the afternoon.

תנן רבי מאיר אומר: אוכלין כל חמש, ושורפין בתחילת שש. רבי יהודה אומר: אוכלין כל ארבע, ותולין כל חמש, ושורפין בתחילת שש.

In light of the above conclusions, the Talmud returns to address the issue of leaven. We learned in the Mishnah that R' Meir says: One may eat leaven the entire fifth hour of the 14th of Nisan, and one must burn it immediately afterward at the beginning of the 6th hour. R' Yehuda says: One may eat the entire 4th hour, and one places it in abeyance for the entire fifth hour, and one burns it at the beginning of the 6th hour.

לאביי אליבא דרבי מאיר, דאמר: אין אדם טועה ולא כלום ⁠— ניכול כולה שית. ולהך לישנא נמי דאמר: אדם טועה משהו ⁠— ניכול עד סוף שית.

The Talmud raises a difficulty: According to Abaye’s explanation in accordance with the opinion of R' Meir, that he said a person does not err about the time at all, let him eat leaven for the entire 6th hour, as by Torah law it is permitted to eat until the end of the 6th hour, i.e., midday. The rabbis issued a decree prohibiting the consumption of leaven from the beginning of the 6th hour, lest one unwittingly come to eat after midday. However, according to R' Meir, people do not err in this regard at all and that concern is unfounded. And according to that version of R' Meir’s opinion, that he said a person errs a bit, let him eat leaven until just before the end of the 6th hour.