The movie takes place in 1932 but some of the songs Ivor Novello sings for the guests didn't come out until years after, like "Glamorous Night" (1935), "Why It Wasn't You" (1937), "I Can Give You a Starlight" (1939) and "Waltz of My Heart" (1939).
Lady Trenthem contends that Ivor Novello's film, The Lodger, was a flop. Although Novello humors her by agreeing, the film wasn't a flop. This was done to convey that Trenthem and the other aristocrats are not movie-goers (seeing as how she initially mispronounced it as, "The Dodger").
When Lady Trentham is getting ready to leave, she doesn't have a scarf around her neck. When she goes to the vanity, she throws the scarf to Mary, but then in the next shot she is taking the scarf off.
When Mable informs Lady Trentham that she doesn't have a lady's maid, Mr. Nesbit lays his tea cup and saucer on the mantel. After the scene cut, when Lady Sylvia joins the group at the table, Freddy again places his cup and saucer on the mantel. In the same cut, a tea cup in the hands of Lady Trentham suddenly appears.
When Mrs. Wilson enters the Servants' Hall while the inspector is telling the staff that he will be leaving, Mrs. Croft exits the room but the door remains open behind the Inspector. When Mrs. Wilson leaves the room, Mrs. Croft reenters, the door is shut and she opens it.
When Mrs. Croft and the cook are counting the knives, we see servants bringing the candelabra from the dining room down the stairs. Then we see the gentlemen in the dining room, where the candelabra are still lit.
At the beginning of the movie when Constance walks to the car, she is wearing pointed-toe shoes with a heel. When she arrives at Gosford Park you can see, as she enters the doorway, that she's clearly wearing "flats".
In the beginning, in the car, when Mary turns back towards Constance to "see what she wants", you see Mary from Constance's point of view and she is turning to look over her right shoulder. Then the camera jumps to looking towards Constance from next to Mary, from the front of the car, and now suddenly Mary is looking at Constance over her left shoulder.
The goof items below may give away important plot points.
During the build up to the murder sequence, with Ivor Novello playing piano, Morris Weissman clearly leaves the room "to use the telephone". We see him at the telephone, then he is back in the room again during later shots, but when the murder is discovered, he is still on the telephone.
When McCordle's coffin is shown loaded into the hearse, there are only two men closing the back of the hearse. There are no other pall bearers in sight. It would require more than two men to carry a coffin with a man the size of McCordle in it out of the house and to lift it into the hearse. Only two men trying that would find it impossible.