When Derek rides Avatar for the first time, the horse has no reins or bridle. When he returns, it has both.
Throughout the film, a character's voice will suddenly change, even in mid-dialog. (Probably the work of voice specialist Paul Frees.)
LeBorg tells Lady Jocelyn that the wine and oysters aided his recovery, but we've seen nothing of wine and oysters. Clearly a scene was cut, as this line makes no sense in this context.
When the Rev Mr Atley says he hopes he is not overweight, the attendant says he is over by three pounds but the shot of the scale shows the weight is in kilograms.
A hay rake was put next to the rock wall by the killer after dragging the fox in the bag around the hunting area. In a later shot near the end of the fox hunt, the rake is pulled away by a tractor. This is done under the orders of Gethryn, who had previously discovered the hay rake. After the dog sniffs out the killer, who hadn't seen the movement of the rake, he jumps a horse over the wall and is killed on the hay rake as he falls from the horse.
At 0:50:19, Mrs. Karoudjian refers to her husband as "Mr. Kadjourian". The mispronunciation was done to emphasize her contempt for her husband's command of the English language; Gethryn is careful to pronounce "Karoudjian" correctly.
The closer shots during the fox-hunting scenes were shot in a studio using rear projection. In these shots, wires can be seen holding the actors and allowing them to move up and down as though riding real horses.
There is a noticeable crease or wrinkle in the back of "Mr. Pythian's" bald head, which gives away the fact that it's part of his disguise.
Gwendolynne La Doll's address is shown on the manuscript as being on Fulham Road, London S.W.8. Later dialogue describing the location as "the Fulham Road" makes it clear that the main street has that name. But this runs from Fulham to Brompton, whereas the SW8 postal district lies across the Thames in districts such as Battersea and Wandsworth.
When Jocelyn confronts Mr. Pythian in Messenger's flat, he's clearly taller than her. But later, when describing Mr. Pythian to Gethryn, Jocelyn says that she "towered over" the man.
When Jocelyn encounters Mr. Pythian in Adrian Messengers' flat, she fails to ask how the "neighbor" was able to enter the flat without a key.