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.
When Charlotte calls upon the Bennets after Mr. Collins' failed proposal to Lizzy, she is wearing a lace cap under her bonnet as she did after marrying Mr. Collins. Unmarried women did not wear lace caps.
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.
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.