When reading eyewitness reports from old battles it's not only the "how do you remember what happened" as with the example David mentioned, but also what motivations do that person have to write the description. What was the practice of the time, of the culture, of the person? What does he have to win / loose from a correct or "enhanced" version of the events? Would that person admit failure even to himself? How big is his ego?

I haven't studied Waterloo more than any other wargamer but I have studied some of the battles of the Great Northern War and Tsar Peter the Great lied copiously in all his descriptions of battles. The descriptions are easily refuted out of just looking at a detailed map of the battle or looking at battlefield finds or even troop numbers and the Swedish church books recording all births in Sweden. Tsar Peter gave Swedish army numbers way over what the army at Poltava even were at full strength and several regiments were below half strength. I would trust later accounts and the more distant in time and the more distant in modern politics the better. Modern Russian historians are "correcting" history due to politics (Carolus Rex (or Karl XII) fought with Mazepa and Ukrainians against Tsar Peter).