to events that JA doesn't describe? In Chapter 6, Elizabeth mentions that Jane has dined in company with Bingley 4 times -- those might have been evening parties which Mr Bennet attended. So it is possible that Mr Bennet said something or acted in some way as to be noticed by Mr Darcy in a negative light.

The only public incident that we read about, I believe, is at the Netherfield Ball when Mr Bennet tells Mary to leave the piano for some other woman to play. This may not have been awful, but it was abrupt and obviously embarrassing to Mary.