Kinnear's LandRover [BYX 564B], driven by Eric Paice throughout most of the movie, is the same vehicle used by the Police when they raid Kinnear's mansion near the end.
When Brumby is at the top of the stairs telling people to get out, some people run down the stairs but in the next shot, no one is running out of the house.
Carter's rented Cortina is actually 2 cars, which have different registrations YBB 371H and YBB 372H at various points during the film. 371H acquires a dent above the front grille at one stage, while 372H never shows this dent, but does lose its front bumper.
When Jack Carter arrives at Cliff Brumby's house you can see people at the party through the window. As the car door opens and he gets out, the people visible through the window have changed.
The white Sunbeam Alpine may be two different cars, one with front mudflaps and one without. Also in the interior footage as the Alpine is driven round the car park, all windscreen fittings such as sun visors and hood clips have been removed but are visible as it falls into the river.
When Carter is shot by the sniper, he is facing the water, but the sniper shot hits dead center forehead. The angle of his body in relation to the angle of the shot is entirely askew. The bullet would have entered Carter's head at the right rear.
In the final chase scene Carter starts chasing his brother's killer on foot from Blyth Staithes and eventually catches up with him on the beach at Easington Colliery some 15 miles away in a direct line. The Quayside however is on the North side of the River Tyne and Easington is South of the Tyne with the only bridges in Newcastle so the only way to get there is to go into Newcastle, cross over a bridge and head back to the coast which is a good 30 miles.
In the scene with the girl driving/having sex with Jack, they show the car's speedometer and the mph are impossible -- it jumps from low speed to 100+ mph in an instant. Either the speedometer was broken or (more likely) it was an effect done by spinning the input shaft with a drill or something like that.
While Carter is being driven by the girl in the Sunbeam Alpine apparently at great speed, they are overtaken by a sedately driven dark blue Hillman Minx in the background.
(at around 1h 14 mins) After Carter has dunked Glenda underwater in the bath and pulls her up again, his shirt is soaking wet. As there was no big splash and as Carter never fell into the bath at that time, there is no need for his shirt to be that wet.
In the first scene where Jack is alone in the house with his brother's body, he's seen inspecting his brother's body in the casket. During the closeups you can see the brother's right eyelid twitching.
When Carter throws Brumby off the car park, the Police are on the scene in seconds, however when the shoot out at the ferry is in full flow there is no sign of them.
(at around 1h 2 mins) the screeching of brakes can still be heard, briefly, after the car has come to a standstill.
Tightening of the screws on the coffin.
At around 1:18:30 as Jacks shoves the soggy Glenda into the boot of her car, in the top left of the screen the balding head of the driver can be seen hunched over trying not be seen in-shot. As Michael Caine didn't drive (manual transmission at least), this is presumably the person who drives the car away.
Shadow of camera clearly visible after Carter slaps Margaret on the bridge.
As Carter (Michael Caine) checks each stall in the men's room while searching for Thorpe, one can see on the wall at the end of the men's room the shadow not only of Carter, but also that of the cameraman filming the scene.
In the ferry terminal shootout sequence, during the dialogue between Carter and the henchmen, a cameraman is reflected in the window of the ferry.
When Jack goes to his brother's house and is inspecting a room, you can clearly see a crew member's reflection in the dressing table mirror.