What is the ruling on someone who commits an act, being ignorant that it is a sin, and does not find someone to consult until a period of time has passed during which he might forget what he did?

If a layperson cannot find someone to ask for a fatwa (definition) and forgets what they did, there is no sin upon them. Ibn al-Qayyim says: "If a layperson encounters a new situation and is in a place where they cannot find anyone to ask," then they must fear Allah as much as they are able, and strive to seek the truth to the best of their ability. If they are unable to find any indications that would lead them to a preferred ruling for that situation, then the obligation falls away from them, and they become like one whom the call has not reached.