Other mistake: When Andie goes over to the diamond display (at the party) she isn't wearing any jewelry, and touches her ears a couple of times, demonstrating there's nothing on them. Later on she is seen wearing 'Isadorra' (the yellow diamond necklace) and a pair of matching earrings, which are certainly large and sparkly enough that they would have been visible if she was wearing them earlier. When security chases after Andie after she leaves, they only take back the necklace. I guess that the diamond earrings weren't that important. (01:39:30)
Other mistake: In the scene where Spears is walking down the street and stops at the magazine stand to buy Composure Magazine, she hands the man a bill. The seller gives Spears her change without even looking at the money he was handed. How does he know what the exact change should be?