Other mistake: When Perseus wakes up in the stadium and he is talking to the old man, you can see a flesh coloured pair of shorts under his tunic.
Other mistake: After Kalibos releases the scorpions to attack Perseus and the men, he starts wailing on the horses with his whip. He is obviously standing a considerable distance from the horses, yet shots reveal the whip striking a few of them.
Answer: To me, it just looks like the way the prop catches the light as the actor lifts it up, and then higher up, the angle of the prop isn't catching the light in the same way.
Bishop73