In the wedding scene, Colonel Brandon wears a military coat - but it has only a single epaulette, which was the mark of a junior officer, no higher rank than captain. And it has gold buttons and silver lace loops on the lapels, which is an impossibility in British military uniform where buttons and lace are always the same colour, either both silver or both gold. (Mixed metals colours were known in French military uniforms, but never British ones.)
The film is set around 1810 (the original novel was published in 1811). There are a pack of dogs, one of them a Nova Scotia Duck Tolling Retriever, near a gate. Nova Scotia Duck Tolling Retrievers were developed in the early 19th century to lure waterfowl. The purebreds originated in Nova Scotia, Canada. The Nova Scotia Duck Tolling Retriever was not introduced to the UK until the 1980s, and was not fully developed as a breed anywhere in 1810. This dog is not a golden retriever, as can be evidenced by the smaller stature, the orange/red fur and the white markings on the chest and nose.
Among the several happy working and hunting dogs running around the Barton Park Estate, one is very obviously a Golden Retriever. However, the 'Flat Coat Golden Retriever', as we would recognize it today, was not bred until the 1880's. The breed was first accepted by the The Kennel Club of England in 1903, and was not officially named 'Golden Retriever' until 1920, more than 100 years after the setting of the movie.
The scene at Norland where Edward is reading the poem is set in the evening. The following scene where the Dashwoods receive the letter from Sir John Middleton occurs in the afternoon. The next scene between Marianne and Elinor in the bedroom occurs again at night but is referred to as if it had happened earlier the same evening.
When Colonel Brandon announces the picnic he says it will be on "Thursday next". Later, on the day of the canceled picnic when Marianne agrees to meet Willoughby the following day she says that she will "stay behind (home) from church" on what would be Friday. It is unlikely that there would be full congregational church services held on a Friday rather than on a Sunday.
When Marianne and Elinor return from London and join the Palmers at their estate, Marianne walks to the top of the hill to see Willoughby's estate. Elinor is inside by the window. First Mr. Palmer hands her a cup of tea, which she takes from his hand. The next shot is of Marianne, and then cuts back to Elinor. She no longer is holding a cup, but Mrs. Palmer pours another cup and hands it to her.
In the dance scene when Elinor ducks between the dancers on first pass she is wearing a shawl low on her arms. When she comes back between the same row of dancers only a second later she is not wearing the shawl.
When Marianne and Margaret stand on top of the hill in the rain, Marianne's dress is wet. When she runs down the hill, it is suddenly mostly dry. Then when she trips and falls down, her dress changes to totally sodden through and dirty between shots.
The goof item below may give away important plot points.
The scene in which Lucy Steele reveals to Fanny Dashwood that she (Lucy) and Fanny's brother (Edward) have been secretly engaged initially shows Lucy stroking a dog. When Fanny realizes what Lucy is saying, she becomes hysterical and sends everything flying, but the dog is no longer there.
At the end of the film, as Edward is coming up the road, Elinor is working in the garden; her hands grimy up to the wrists. As the women rush into the cottage just seconds later, the amount of dirt on her apron doesn't match the dirt that was on her hands, and her hands are practically clean.
When Elinor and the rest of the family stands when Edward comes with the news of his brother's wedding, first Elinor's hands are at her sides, then they are under her apron (presumably to hide her dirty hands), and then they are back at her sides.