The registration number of the taxi the young Libby leaves in is, at first, CYP 44V - when her bags are thrown in. This is recycled footage from shots of the same vehicle (CYP 44V) which we see the older Libby and daughter Daphne leave in towards the end of the film. Additionally, as the younger Libby leaves, after her bags are thrown in, the registration number changes to MMX 902L (a vehicle seven or eight years older).
Lord Orwood claims that the chandelier was given by Napoléon Bonaparte to Josephine after the Battle of Borodino. The Battle of Borodino took place in 1812. Napoleon had divorced Josephine in 1809. He would have given the chandelier to his wife Marie Louise, whom he married in 1810.
In front of the portrait of his ancestor, Henry tells Daphne that Field Marshal Bingley Dashwood lost an arm at the Battle of the Nile. Field Marshal is the highest rank in the British army, but the Battle of the Nile was a naval engagement.
In the Invitation inviting Daphne to Peach and Pear Orwood's Coming Out Ball, the invitation reads "Miss Peach and Pear Orwood invites..." whereas an actual British society invitation would read "the Misses Peach and Pear Orwood invite..."
When Daphne meets Ian, she asks if he's playing a Gibson J200 guitar, and he replies affirmatively. We never get a complete shot of the guitar, but we can see that it's a cutaway model - the Gibson J200 does not have a cutaway.
When Henry calls Libby right after Daphne arrives, he dials her number without using Directory Assistance - even though he hasn't seen her in over 17 years. Also, he doesn't press anywhere NEAR enough digits for an overseas phone call.
Daphne's bedroom is obviously not part of the house we see from the exterior. Daphne's room's interior shows it as a corner room (a window is on the long wall) with a bay window. But the Dashwood house is a flat-front, and When she and Glynis and Clarissa talk to Lord Dashwood out their windows, Daphne is supposedly in a room in between the other two, with a single French window opening onto a balcony.
When Ian first comes to Dashwood Manor and is arguing with the security guard about having an appointment to see Daphne, the size of the gap in between 2 of the gate's metal bars changes from wide during shots of Ian speaking to very, very thin during shots of the guard speaking.
When Armistead and Daphne are on the dock at the party, right before Daphne pushes him into the water, Armistead starts rubbing Daphne's arm with his right hand. But in the shots from behind Daphne, it's obvious that Armistead is only miming the action and rubbing his hands in midair.
in the last scene when she is dancing with her dad and having conversations her left cheek is touching his left shoulder and cheek but as the conversation progresses the camera shots keep changing and her right cheek is touching his right cheek, back and forth and back and forth again and again.
When Henry, Glynnis, Clarissa, Alastair, Daphne, and Jocelyn are discussing where Daphne should stay, to avoid the scandal, Jocelyn turns slowly to the right to face Daphne. In the next shot, when she says "Oh, the girl must stay here", she is completely facing Daphne, with no true indication she had completely turned in the split second.
Both Henry and Daphne are referred to incorrectly. As the hereditary Earl of Wycombe Henry Dashwood would not be referred to as 'Lord Dashwood' as he is in the film, but 'Lord Wycombe' whilst Daphne, as the daughter of an Earl would not be referred to as 'Miss Daphne Dashwood' But Lady Daphne Dashwood. In fact it states early in the film that Henry has given up his seat in the House of Lords, but as hereditary seats are attached to peerages, not individuals, the only way for him to do this would be to give up the peerage altogether, and as such become 'Mr Henry Dashwood'.
All invitations from Queen Elizabeth II are commands, forwarded by the appropriate member of the royal household. As such when the film says 'Her Majesty the Queen invites you to the Annual Garden Party' in reality the invitation would read 'The Lord Chamberlain is commanded by Her Majesty to invite Lord Dashwood and the Honourable Daphne Dashwood to a Garden Party'
When Daphne is introduced at her first function, the introducer is wearing 4 miniature medals. Two WW2 stars and two WW2 medals. His medals are mounted incorrectly. His Defence Medal and his 39-45 Medal are the wrong way round.
During Henry's press conference near the end, there is a shot behind his head showing the entire audience. On the far wall opposite Henry, two Union Jack flags are hung vertically. The one on the right is hung upside down. The proper way to hang the flag dictates that the thick white stripe in the upper left-hand corner is on top of the smaller red stripe, as correctly seen on the flag on the left.
In order to move in any direction a boat needs a means of propulsion, in the case of a rowing boat that will be oars. After falling into the water Daphne and Ian climb back on the boat and start making out lying down at the bottom. No oars, no person rowing. In the next shot, while they are still on the bottom, the boat is moving forward under the bridge, which is physically impossible.
Much is made of Henry disclaiming his seat in the House of Lords as a hereditary peer, something that was not possible to do until 2014 without giving up the peerage entirely. However, the House of Lords Act 1999 removed the right of hereditary peers to sit in the Lords, mooting the issue. (Only the Lord Great Chamberlain and the Earl Marshal, important ceremonial figures, still sit by right.) Hereditary peers now elect ninety-two of their own to sit in the House - a peer who does not wish to sit simply does not stand for election. Barring unusual circumstances, Henry would have never had a seat in the House of Lords to give up.
While the issue of Henry disclaiming his seat in the House of Lords while remaining a peer is almost certainly an error, the scenario laid out in the film is possible, if rare. While hereditary peers lost the right to sit in the Lords in 1999, a hereditary peer who also holds a life peerage (and some do exist) has a seat in the Lords by right as a life peer, and is ineligible for election to the Commons. In this rare situation, Henry could have disclaimed the life peerage, giving up his seat in the Lords, while retaining his hereditary peerage.
At the New Jersey wedding in the beginning of the film, during the father-daughter dance, there is a shot of the bride's back where you see that only about 5 out of the 30 buttons on her dress are actually buttoned.
When Henry shows up at the wedding to surprise Daphne, you can briefly see Ian getting out of the boat as well. You actually see him twice. Then later her father says he brought along a large present: none other than Ian.
The goof item below may give away important plot points.
When Henry arrives at his press conference to withdraw his candidacy, Clarissa is wearing a solid brown barrette on the back of her head. After Henry punches Alistair in the face and Clarissa is kneeling over him, she is wearing a different barrette.