Humans have much more confidence in their memories than is really warranted. Do you remember any really interesting facts about the Titanic sinking, for example? I certainly had a few memories of it which I assumed were probably all true (clearly Leonardo DiCaprio wasn't actually on it when it sank...) because presumably I kept those memories from textbooks and such, right? Not so much. After reading a bbc article on the sinking and the myths that surround it I was forced to conclude that what I really knew after seeing the Titanic movie and reading many articles and books was pretty close to zero in terms of substantive fact. I had plenty of ideas swirling around in my head but my ability to sift the actual facts from movie tidbits has vanished. Even if my memory for data was perfect, which it most assuredly is NOT, then my memory for the sources of my data is so full of holes that I can't trust anything I recall.
I recall reading recently a study that looked at eyewitness recollections of events. Researchers got people who witnessed an event and who were convinced they would remember it forever to answer questions about it and then followed up much later on to find out how long their memories would remain. After 16 months their answers to questions about the event had degraded to the point that they only got 50% of the answers correct. It is pretty reasonable to characterize their memories as no better than random guesses at that point even though their confidence in their memories was almost entirely intact. And this was for an event that the observers were sure would stick with them; how bad must our memories be of events we didn't ascribe much importance to at the time?
Even if our memories didn't degrade we would do well to disbelieve eyewitnesses. They have an unfortunate tendency to fill in gaps with assumptions and guesses which are often informed by events that happen afterwards and vastly overestimate how accurate their understanding of the situation was. I think this is largely due to a very serious misunderstanding of our memories; people often think of memories as pictures that are stored in our brains but they really don't work that way. We don't have a matrix of pixels filled with colours but rather a collection of emotional impressions and fragments of ideas jumbled together. We might well remember a striking colour or object but even that simple idea will often be corrupted or mistaken; we really do remember emotions with associated objects and not photographs.
I find it funny that sometimes I have memories that are truly flagrant lies. I remember stories (like the story of the groinpost, for example) from university and I recall the incident. I remember the whole scene... except I was never there. I have heard the story told so often and so vividly though that my mind has constructed the entire thing and the memories of it are just as real for me as the people who were actually there! It makes me really grateful for modern police investigations; they aren't perfect but I shudder at what masqueraded for justice back when an eyewitness who saw the event in bad lightning at long distance (and who today would get glasses) was considered *good* evidence.