When at the Netherfield Ball, and Mary starts her second song at the piano, her father is seen to get up from the table to stop her. You can see Mary at the piano in the background, but, despite the audio implying she is supposed to be singing, her mouth is not moving and remains closed throughout that entire shot.
At the beginning of the Netherfield dance where Lizzy dances with Mr Darcy as the music comes on, the camera is on Lizzy. That shot of her must have been too fast because they slow it down considerably, shown by a lady fanning herself in the background, and Jane further down the line nodding to someone on the opposite wall, both in slow motion.
When Mrs. Hurst is playing the piano at the Netherfield Ball, the hand that showing the musical accents, is the wrong hand for the melody. She is accenting the upper portion with her left hand, which would actually be the lower notes.
The tune to which God Rest Ye Merry Gentlemen is sung at the Phillips' party wasn't published until 1833, 20 years after Pride and Prejudice and perhaps thirty after the intended setting of the novel.
When Mary begins to sing her second song at the ball at
Netherfield, there is a shot of her, and her mouth isn't moving at all even though the singing can be heard.
During the Netherfield Ball scene, the reflection of the camera crew's lights can be seen in the mirror of the ballroom.
When Mr. Darcy and Elizabeth dance for the first time at Netherfield, there is a short black hair trapped in the camera.
At the ball at Netherfield, Lydia calls Denny 'Danny' when she pulls him away from Elizabeth.