Triangle

When Jess awakes from her dream on board the yacht, you can see an electrical socket, which looks more like one used in Australia, even though it was set in USA.

When writing the "kill them all" note, the over the shoulder shot shows the pen being held with index finger tucked well under her thumb.

Face on, the pen is being held in a totally different pinching style.

When Jess wakes up below deck Heather asks her if she would like a glass of champagne, Jess smiles and says yes.

But when they both climb to the deck moments later Jess is empty handed and Heather walks away with both glasses.

When Jess is driving with her son, something hits the windscreen causing an obvious splatter of blood which visibly fills the windscreen (1:27:18) yet when she pulls over (1:27:21) there's only a stream of blood.

Cars are shown with front license plates.

However, cars in Florida do not have front license plates.

Additionally, the "Florida" plate design in the film is different from any actual issued license plate.

Towards the end of the movie, when Jess prepares to drive off she puts the duffel bag containing her doppelgangers corpse in the trunk.

During the accident scene, the doppelgangers corpse is laying on the pavement, no longer in the duffel bag.

The positioning of the corpse in the road is deliberate, as Christopher Smith explains on the DVD commentary.

The audience is meant to question whether the "doppelganger" has really been thrown out of the trunk and bag during the crash, or was in fact the "real" Jess who was driving - and therefore killed - in the "original" accident.