When Veronica takes a selfie on Sierra's phone, she has her empty hand to her mouth. However, when Sierra looks back on the photo later, Veronica is holding her bra strap in her hand up to her mouth, which wasn't there when she was taking the photo.
When Jamey phones Sierra, her phone shows two buttons (red and green) to either decline or accept the call, but a few seconds later her screen has changed to a 'swipe to answer' button.
Jamey, who has an Android, asks if he can Facetime with "Veronica", who has an iPhone, but in reality, you cannot Facetime between an Android and an iPhone.
While Jamey (Noah Centineo) is playing games with his brother, he has the XBox controller on his lap. On that specific controller, the light would be on if the console itself was on. The light is off when it's on his lap; they cannot be playing video games while the XBox or his controller is off.
When Veronica takes a selfie for Sierra, three film-crew members are visible behind her on her phone camera, most notably the boom operator on the right.