Well a good place to start here would be with Herodotus. Considered one of the earliest "Historian"s, Herodotus pioneered a lot of the techniques still used today. But he is also famous for misconstruing facts, reporting as fact statements made by people about events they never witnessed, and generally exaggerating details in order to paint a more dramatic, livelier picture.

For instance, during the battle of Thermopylae (famous recently in the movie 300), Herodotous estimated the invading Persian army to be around 2.5 million, not including support personnel. Its generally agreed that this is a wild overestimation. Most modern estimates put the figure closer to around 70k-300k.

In fact one of the running themes in ancient historical accounts is the wild exaggerations made in regards to the number of soldiers fighting in any given battle. Right up until the Napoleanic era scholars, writers and historians would vastly exaggerate or downplay the number of troops involved on each side.

Consider the Battle of Nicopolis in 1396. The number of troops on both sides was heavily disputed, with each side claiming to have "been outnumbered by 2 to 1". The numbers nowadays are estimated to have been around 15k on each side. Johann Schiltberger, who faught at Nicopolis, estimated the European strength to be around 16k, while the Turks supposedly fielded an army of 200k. This is, as I said, part of a long running tradition in ancient writings, and something anyone who studies ancient sources needs to keep in mind.

edit: Another example would be estimates of the number of Mongols fighting in Ghengis Khans army. Esitmates frequently were in the millions. But this is now understood to be a result of the Mongols ability to attack one side of a kingdom, "disappear", and attack in a completely different area. People thought they were facing an enemy numbering in the millions, but the reality is that the number of actual Mongol cavalrymen probably didn't number greater then 100k. At the seige of Baghdad for instance the Mongol cavalry presence is thought to have been around 40k. This of course doesn't include the huge number of soldiers that fought (unwillingly or otherwise) for the Mongols.