In Mosiah 27:22-23 we read about the priests who fasted for Alma after seeing an angel:
And he caused that the priests should assemble themselves together; and they began to fast, and to pray to the Lord their God that he would open the mouth of Alma, that he might speak, and also that his limbs might receive their strength—that the eyes of the people might be opened to see and know of the goodness and glory of God.
And it came to pass after they had fasted and prayed for the space of two days and two nights, the limbs of Alma received their strength, and he stood up and began to speak unto them, bidding them to be of good comfort:
This scripture explains that the priests fasted for two days and two nights but when Alma the Younger is recounting his conversion story he says in Alma 36:16:
And now, for three days and for three nights was I racked, even with the pains of a damned soul.
Why would Joseph say the priests fasted for two days and two nights if Alma was racked with the pains of a damned soul for three?
The answer is given in Helaman 9:10
And it came to pass that on the morrow the people did assemble themselves together to mourn and to fast, at the burial of the great chief judge who had been slain.
The fasting did not begin until the day after the chief judge had died, implying the Nephite tradition was to begin the fast the day after.
Wouldn’t it have been easy for Joseph Smith to simply say that the priests fasted for three days? How did he remember this added complication?