Deliberate mistake: The Americans actually spoke with American accents, but these very new Americans were British in origin, and so they couldn't develop a new accent in such a short time. The Americans should still have British accents. This would have been confusing and is done for the viewer's benefit, but still a mistake.
Deliberate mistake: Jimmy catches a fly in the air and puts it in his mouth. He then spits it out with great power. You then hear a "Bzzzzz." assuredly made by the fly. How can a fly flap its wings when spit out in that velocity? (00:48:30)