When Emily and Stephanie meet for the first time in Emily's house, as Emily removes the fabric cuffs from her wrists, she removes the cuff on her right wrist twice.
Sean and Stephanie are talking over a couple of glasses of Laphroaig scotch. At one point, the glass disappears from Sean's hand and then in the next shot reappears.
When Stephanie Anna Kendrick is at the painter's Linda Cardellini studio, the knife in the painter's hand changes sides once from having the sharp edge to the left to the right, then the left side again.
The police discover after Emily goes missing that she rented a car from Budget paying with cash. Budget Rent-A-Car does not allow vehicle rentals without payment via credit or debit card, nor do any others in the US. Further, they require a valid driver's license as well, and thus would've already had a record of Emily's rental even if she didn't use a card to pay for it.
Stephanie's Subaru spins out of control when she applies the brakes hard. All modern vehicles have anti-lock braking and stability control to specifically stop such an occurrence. Furthermore the Subaru depicted has all wheel drive and torque vectoring which improves traction and makes such a spin all but impossible without disabling critical systems.
When Sean's phone screen is visible as an inbound call rings, the wireless carrier shown on it is Rogers. The company only provides phone service in Canada, where the movie was filmed, not Connecticut, where the scene takes place.
When Stephanie receives the email about Emily being spotted in Michigan, you can see it is actually an email that has been typed but not sent. The shot of the computer screen shows the "Compose" tab of Stephanie's email is open (not the inbox), with the "send email" button at the bottom of the page.