Continuity mistake: In Jack's first appearance to David in the hospital room, Jack uncovers the food on the tray and takes a piece of toast from another plate on the tray. The silver cover on the tray changes position throughout the scene in various shots. Sometimes the cover is resting partly on the tray and sometimes flat on the tray table surface.
Continuity mistake: In the scene where David (as a werewolf) exits the theatre, causing general havoc and mayhem, a car crashes into the one in front of it, sending the "driver" through the windshield. If you watch closely (or use slo-mo), you'll see that the driver goes through the windshield just before hitting the car in front of him (or her).
Visible crew/equipment: The couple in the street are the first to be attacked by the werewolf in London. As they walk in the street past the houses, you can see a large Studio Light reflecting in the windows.
Factual error: Though it's set in England, hospital personnel use American terms like "charts" and "orderly". In any British hospital these should be "case notes" and "porter", respectively .
Plot hole: The first time David werewolfizes, he's reading a book in the brightly lit apartment. Exterior shots of the full moon show it's nighttime out, so the lights inside the apartment must be on. But when Dr. Hirsch calls to check on him, the shot is of the phone ringing, unanswered, in the completely darkened apartment. That's considerate, David. Even as a werewolf, he must have run around turning lights off before going on his rampage.
Continuity mistake: When David has his first transformation watch as his feet first start to change. His big toes grow and his pinky toes (what I call them) stay in place. Then when it shows David on his back. His pinky toes are the same length as his index, middle, and ring toes but his big toes haven't changed position just shape.
Continuity mistake: When David begins his first transformation, his nails turn into claws. At the end of the movie when David is sitting in the theater and going through another transformation, claws are seen sprouting from his nails.
Continuity mistake: Jack's friend, who he went to England with, appears to have a magic jacket. It changes colour twice during the movie: from blue to brown to green.
Other mistake: When David changes into a werewolf for the first time you can see his huge feet vibrate when he moves his legs in agony: they are made from rubber.
Visible crew/equipment: Once the doctor arrives at the pub in Yorkshire to 'investigate' it (as usual) rains. It is plain visible that the rain is water sprayed from several hoses.
Revealing mistake: The first time David werewolfizes, we're treated to the complete process... hair developing, bones painfully lengthening and twisting... and the torso of the actor very visibly halfway in the floor while all this is going on to the full-body werewolf prosthesis. Oh for computer graphics at the time to erase that detail of the process.