Continuity mistake: In several episodes of the early seasons, Carson is said to have once been part of a music hall song-and-dance team with Charlie Grigg, who comes to Downton to blackmail Carson. But in later seasons, Carson is said to have come to Downton at a young age, serving as a hall boy.
Continuity mistake: Every letter's handwriting looks the same - no matter who wrote it.
Episode #1.1 - S1-E1
Continuity mistake: Near the end of the first episode, when Robert Crawley gets into bed he first puts his candlestick down on the nightstand. Then it disappears. Then when he's in bed and ready to blow out the candle, it's suddenly back again.
Episode #1.2 - S1-E2
Continuity mistake: When Cora (Lady Grantham) goes to Violet (the Dowager Countess)'s house to discuss Matthew and Isobel Crawley, she sits at the table and takes off her gloves shown in a shot from the front, and then the shot changes to over her shoulder and she is seen taking the same glove off again. (00:37:25)
Episode #1.7 - S1-E7
Continuity mistake: Near the end of the episode with the garden party, there is a scene where Edith is pleading with Anthony Strallan not to leave. You can see Lady Grantham lying down on a lounge chair in the background, with a man who is distinctively Lord Grantham standing next to her talking to her. But the scene immediately cuts to Lady Grantham, her maid O'Brien walks over and Robert is nowhere to be seen.
Episode #2.4 - S2-E4
Continuity mistake: Mary has written a letter to Matthew. We hear her voice reading the text, which begins: "So there we have it. I look forward to introducing the two of you, just as soon as you are next at home..." Next, when the page of the letter is shown, it reads "...happy to hear - come to pass. I look forward to introducing you to Sir Richard, just as soon as you are next at home." (00:11:03)
Christmas Special 2012 - S3-E9
Continuity mistake: In this episode, Cora tells Mr. Gregson that she started reading "The Sketch" when Edith began to write for him. In Season one, episode one, Carson tells William to iron "The Sketch", because "her ladyship only ever reads the Sketch."
Episode #5.6 - S5-E6
Continuity mistake: When Lady Mary is competing in the Point-to-Point horse race, she is riding her mount sidesaddle style, wearing a long-skirted riding habit. When she (actually the stunt rider) jumps a hedge, she is now riding astride, with both legs over the horse and using a two-stirrup saddle. Also regular riding breeches (pants) are underneath the long skirt when it flies up a bit. These would never be worn with this type of lady's riding habit.
Episode #6.7 - S6-E7
Continuity mistake: When Edith stops by her office Laura's there and she's holding a cigarette, which keeps switching between her left and right hands while the two women talk.
Answer: Lord Sinderby purchased the house, furnished and appointed, from someone else, complete with staff that come with the home (some of whom aren't happy to be working for a Jewish family). He bought the title, as well. If the artwork featuring Jesus is valuable, he might keep it. Also interesting, his ancestors Anglicized their name, though he is proudly Jewish.
Michael Albert