When Will and Lou reach an edge of almost the top of the castle, a front close-up shot to their faces shows calm air while in all other scenes, it is windy. That shot must have been taken on another day.
When Nathan is lifting Will into his chair (in a scene where Lou comes, talking about horse racing) Nathan places Will's left arm on the arm rest twice in two sequential scene.
When returning from the trip, there's a scene with Lou and Will at a check-in counter at the airport. When flying with a private carrier, you don't go to a check-in counter, just pass through a separate security and ID control, and a private handler takes you to your jet at the apron.
It looks in the film as if Will has tried to cut his wrists in a suicide attempt, though this would be physically impossible to do this due to his quadriplegia. It is in fact explained in the book that he ran himself in his wheelchair into a sharp edge multiple times.
During the scene where Louisa watches the birthday video on Will's Mac, the left side of her face noticeably deforms from bad keying/roto due to the green screen.
When she's trying on dresses for the concert in her room, Lou's movements don't match what her computer's monitor shows her doing.
At 27:43 when Will says, "I'm not laughing at you", his lips don't move, nor for the "at you" part.
Living in a small town so close to such a wealthy family, there is no way that Lou wouldn't have known of the Traynors, or that their only son had become a quadriplegic.
In a scene in Lou's family's kitchen, her dad talk about finding a "wrench". In England those tools are called "spanners"