Continuity mistake: Caligula orders his chief gladiator to kill a prisoner so he can test the Robe's "powers". The gladiator stabs the prisoner with a dagger and straightens up. Look at the dagger and you'll see no blood on it.
Suggested correction: This may not be a mistake. It's entirely possible to stab either a person or an animal with a quick thrust and retrieve and show very little or no blood. Especially with a slim blade.