The bruise on Frank's left cheek changes notably between scenes,
At 44:44 Frank grabs a clear light bulb from the cupboard. The next shot shows him screwing in a frosted light bulb.
When first questioning Dr Sangster, Lance Phelps folds his arms twice in rapid succession between shots, without enough time to unfold them elapsing between shots.
The guy from DEA would NOT carry the vial loose in his pocket; not likely at all; but if he did, it would be in an evidence bag. He'd also be wearing a suit.
When Frank enters the bathroom for the first time, the reflection of a cameraman is visible in the shower door.
A real pharmacist would have phoned the doctor to verify the drug and amount before filling the prescription.
On all drug vials in the movie, "pharmaceuticals" is misspelled "pharmacueticals."
When the DEA agent arrives at Frank's office, he explains he is from the Drug Enforcement Agency, despite the fact that DEA stands for Drug Enforcement Administration.
A doctor/dentist would not refer to patient records as a 'file'. The terminology is 'chart'.
When Frank orders Bourbon at a bar, the bartender offers him Four Roses, Jim (Beam) Black, V.O., and "Seagram". Only 3 of the 4 are Bourbon. The bartender pours V.O., which is actually a Canadian Whisky.