At the end, when Robin runs through the crowd with his men, his trademark bow and a small satchel suddenly appear in his hand only a couple seconds after a prior shot, when his hands were empty.
During many of the castle scenes, you can hear the rubber soles of the actors' shoes squeaking on the floors.
After the Sheriff feigns death, he says he's "just joshing. GUARDS!" According to Answers.com, the phrase "Just joshing" wasn't invented until the late 1800s in the United States.
When Will and Luke are trying to escape quarantine, at one point they throw themselves onto an adjacent rooftop and you can see that Will's boots have modern, rubber soles.