When the team drives up to the marina with Parker driving the taxi, she throws her cap into the front seat - but she misses, and it falls on the ground next to the car. When she and Hardison return to the taxi, the cap isn't there.
When Nate orders his flight of reds (wines) at the restaurant, the waiter starts unloading a tray of wineglasses onto his table. He unloads all but one, then the camera changes and there are two back on the tray again.
Nate offers his credit card to the waiter for "Lady Charlotte's" bill. Then he 'charms' Lady Charlotte and they walk out seconds later, without his credit card being visibly returned. Although there is a small possibility that the waiter could have chased him, the time needed to run a card through a EFTPOS machine would have meant they would have been out the door before the waiter even reached the cashier.
Sophie identifies herself to Meredith as 'Lady Charlotte..." Meredith immediately says that Sophie is a Duchess. The holder of a peerage in her own right or the wife of a peer is addressed and titled in conversation as 'Lady X" Thus Sophie could have been the holder of any title from marchioness, through Viscountess right through to being the wife of a Knight Ordinary. If Meredith was the expert on social graces and positions that she seems to be, she would not have made this simple mistake.