Throughout all wars, tactics, technology and fighting has changed and developed – see more about the advancements in warfare here – and often these tactics have been seen as unfair. Often there are Rules of War, whether they be spoken or unspoken, but sometimes to confuse the opposition and therefore win the war / conflict, these rules are broken.
One of the trick tactics used by William the Conqueror and the Normans during the Battle of Hastings was to fool the Anglo-Saxon Army into thinking they had won! There is evidence that the Normans would pretend to surrender and run away from their opposition in the hopes that the army would break rank in their false sense of security. Therefore, when the Normans reformed and attacked, it was thought that the Anglo-Saxons would be unprepared and fail to defend against the oncoming army.