Deliberate mistake: In the fight scene with the chariots Maximus takes one of the horses from a chariot and jumps on it. The horse already has a saddle complete with stirrups despite the fact it was pulling a chariot and not carrying a rider. (01:23:30)
Revealing mistake: After the first battle in the Colosseum (The Battle of Carthage), Commodus decides to meet "The Spaniard" in the arena. When Quintus (the head of the Praetorian Guard) is telling the slaves to drop their weapons, we can see one of the spectators hiding a plastic bottle of water (right upper corner of the screen). (01:25:58)
Chosen answer: You're probably closest with the suggestion that they're rather stunned at the sheer speed with which Maximus has carved through the opposition. They'd certainly be used to more of a show, so for Maximus to slay all his challengers in less than a minute would take them aback. If anything, the "Are you not entertained" is closer to the theatrics that they'd really expect to see, which would prompt them into cheering him - up to that point, he's not exactly won their favour, even though he's defeated all comers.
Tailkinker ★