Factual error: If you have ever actually seen London then you can see that the route Hallie takes to get to the house would take you all around London.
Factual error: Copyright stamp at the end of credits reads MCMMXCVIII, that is the third M is superfluous.
Factual error: At the hotel, Chessy's door to her room stays open after she goes into the other room to discuss the whole 'dad doesn't know' matter. In real hotels, the guest-room doors have auto-closing mechanisms for fire safety and security. The door would have swung closed and locked Chessy out, but even though this is a 4 or 5 star hotel which would be fully equipped as far as modernity's, it doesn't.
Factual error: Hallie, in London, dials the operator and says she wants to place a collect call to Napa, California. The phone rings once and Annie answers, then begins talking. On such a call, the operator should have told the recipient, "You have a collect call. Will you accept the charges?"
Factual error: After Hallie and Annie send Meredith out on the air mattress onto the lake, Meredith wakes up and there is a bird on her chest. The bird is a Red-Crested Turaco, which are native to Africa.
Factual error: In the scene in which Annie and Hallie are fencing, it portrays it as though Annie does not win until she hits Hallie (effectively knocking her into the water). However, in a real fencing match, Annie would have won within a few seconds after they started. Once a person is backed off their end of the strip, their opponent automatically gets a point.
Factual error: Placing Hallie and Annie alone in an isolation cabin without a counselor sleeping there to supervise is an enormous liability. No camp that cared about its reputation and about avoiding a lawsuit would ever put two girls in such a potentially unsafe situation.