During The Loyal's performance at Oceanside Alt Fest, Cassie goes back and forth between wearing her wedding ring and not wearing it several times, thus showing this performance was a mashup of different takes.
AT 5:45, Cassie is shown wiping the bar when Luke approaches her to talk from her right. When it cuts to a shot from behind her, she's not wiping anything.
At the 55:41 mark Cassie is onstage with her hands on a mic not wearing her wedding band. 5 seconds later she has it on.
At 1:11:30, a shot of Cass's apartment balcony shows the US flag displayed incorrectly. The stars field should be on the left.
The USMC Ford Crown Victoria has Californian "CA EXEMPT" license plates. Since it is owned by the federal government, it should have DOD license plates instead.
At 00:33 Luke's dog tag chain has only one tag on it.
Near the end of the film where Cassie (Sofia Carson) sings to a live audience, you can see that she is lip syncing another song.
Cassie's apartment is meant to be in Oceanside, California, but is actually an apartment called "The Oceanside" in Long Beach - you can clearly see "Long Beach" repeated on blue banners on the fence of the roller rink across the street when Johnathan DeLuca (Johnno) drives by threatening Luke on the phone, at about the 1:16:40 mark.
Cassie, a diabetic, suffers two emergency episodes. In the second, she's clearly become hypoglycemic (low blood glucose). The first is not so clear. She suffers dizziness. Just dizziness, which CAN be a symptom, but none of the more prevalent signs appear -- fast heart rate, shaking, sweating, nervousness, or confusion. Since the attack comes at a time when she's struggling to acquire more insulin, she's likely suffering from dehydration, which can be more of a problem for diabetics than non-diabetics. The story does not make this clear.