I don't know of any examples from Medieval Ages but one particular battle that fits your description occured in 546 BC between the Greek cities of Argos and Sparta known as The Battle of 300 Champions. The two city states agreed to settle a land dispute in by sending 300 of their finest warriors to fight in one decisive battle instead of fighting a war that would cost many more lives. After many hours of fighting only two Argives remained who travelled back to Argos, claiming victory. However the remaining Argives left without noticing one severely wounded Spartan left on the field of battle. Before committing suicide, the wounded Spartan told spectators of the battle that since he was technically the last man standing on the battlefield, Sparta won the battle. In the end, both Argos and Sparta claimed victory and war was declared between them anyway. So much for one decisive battle. Hope this helps even though it isn't an example from the Medieval ages!

Edit: Spelling