When she meets Uncle Matt, Fifi gives him several kisses. The marks from her lipstick appear and disappear on his face between shots.
Aunt Della tells Uncle Matt not to be late getting home because Lord Epping likes having his five o'clock tea. The traditional English teatime is four o'clock, not five--though in fairness, Epping has been established as an eccentric, so this could be a deliberate acknowledgment of that.
The newspaper Carmelita is reading at breakfast has a photo on the reverse side. The bottom corner of the photo is curled up, showing the photo was pasted on the paper, not printed.
During the duel, one of the knives that sticks into a tree can be seen coming out from a slot in the trunk, not being thrown. The slot is also visible in subsequent shots.
Lt. De la Blanc throws a knife to light a match, but the knife is clearly too far away to come into contact with it.
The lodge receptionist refers to the French character as Captain Le Blanc, whilst his card identifies him as a lieutenant.