There is a famous incident where a psychology professor was teaching his class and a man came into the class, walked up to a student, jabbed him quickly in a cloak and dagger sort of fashion, then ran out. The students were all immediately evacuated from the room and the police called. Officers took statements from the students many of which gave detailed descriptions of the knife used to stab their fellow student. Little did they know that their fellow was alive and well only having been "stabbed" with a banana.
It is actually a normal function of our brains to fill in information gaps. If we see or hear something but do not have enough information to work with to "know" what exactly we are seeing or hearing we would easily fall prey to dangers while puzzling out what is going on. So instead our brain fills the gaps for us with its best guess. The same thing happens with memory. Our memories naturally degrade over time and our brains often fill in the missing bits when we contemplate those memories again years later. It works with short term memory too I believe if say you were not paying particular attention to something a moment ago and suddenly realize something important was said you may try to puzzle it out from your incomplete memory and even convince yourself that something was said that was not. If you did not see it before you should look up videos of experiments regarding "change blindness".