The scar that was burned into Taylor's wrist at the initiation is missing in later scenes when she's not wearing her watch.
When Taylor is running and Ethan stops her for the "apology break," he drops his book bag from his shoulder to the ground. Two shots later it's back on his shoulder, but he never picked it back up; then, in the next shot, it's back off when he grabs her arm.
Towards the end of the movie, Taylor Brooks dials 911 on a cell phone to allow the police to overhear Nathan Lloyd's plot, but she never presses the "send" button on the phone, so there is no way it would've connected to 911's service. You hear the beeps for 9-1-1, but no beep for send...
The IP address that sent the email to Ethan (211.68.356.187) isn't a valid address. IPv4 addresses can't use numbers higher than 255 for each octet.