When Derek is getting ready to go to the Christmas party and asks which tie to put on, after the selection is made, his collar is not on straight. In the next shot his collar is smooth, then it's not on straight, then it's smooth again.
When Derek first has Lisa sitting in his office and the camera view is toward Derek, a coffee cup is in his left hand. However, in the succeeding view looking toward Lisa the coffee cup is suddenly in Derek's right hand.
Sharon is shown wearing white platform shoes when the couple first enters their new home. When they get up to the attic, she is wearing patent leather shoes.
When Derek and Patrick are talking in the employees' kitchen, in one shot Derek is holding a bottle of drink, in the shot directly after the bottle has disappeared from his hand.
When Derek and Lisa are eating at the bar before the Christmas party, Lisa approaches the bar and her watch is on her right wrist. When she is shown in later scenes, the watch is on her left wrist.
The chandelier that falls on Lisa should not have been able to kill her, as it's not an out-sized and heavy chandelier covered in glass ornamentation. The chandelier in the film is the kind of modern, minimalist fixture that would only weigh about 15 pounds at the most. If Lisa could survive a 20-foot drop through a glass table, she could easily survive the chandelier falling on her as well (that's not even mentioning that if you look closely at the split second shot of her corpse, the chandelier didn't even fall directly on top of her and there's no gore).
In the later part of the movie, when Lisa is lying on the bed wearing the "14" jersey, the numeral is mirrored. That's because the scene is from her perspective, when she is looking up at a mirror. When the perspective changes, the "14" is not mirrored anymore.
Disappearing lightbulb between camera cuts when Ali grabs the lit lamp and aims it at Beyonce.
When Ali Lalter and Beyonce are having their climatic fight scene near the end of the movie,the former is in bare feet while the latter is wearing high heel boots.It doesn't make much sense the Beyonce didn't simply end the fight by stamping on Alis toes with her boots.
When Lisa tricks the babysitter while Derek and Sharon were out and attempted to kidnap their son. The babysitter shouldn't have answered the door and Derek and Sharon should've given her instructions telling her not to answer the door for anyone.
In the beginning, when the couple are getting out of the car the reflection of the boom mic is visible in the rear window.