Audio/visual unsynchronized: While Millie is outside Beatrix's room we see her shadow against the door and it reveals that her mouth movement is out of sync with the sound.
Errors in geography: The train crosses Arten Gill viaduct in a southerly direction on the Settle Carlisle railway line. She would be going north from London to Penrith (for Keswick), and would cross northwards.
Factual errors: Norman actually proposed to Beatrix in a letter, and her parents never softened their opposition to the match.
Revealing mistakes: In the railway station scene filmed on the Bluebell Railway, the LNWR on the tank locomotive has been very visibly altered from LWSR. The N stuck over the W is a really bodged job for such a close up shot.