In Chapter 18 of Pride and Prejudice, the Bennett family are attending a ball at the Bingley's rented estate at Netherfield. Mr. Collins is speaking to Elizabeth explaining to her that he is going to go and speak to Mr. Darcy:
"You are not going to introduce yourself to Mr. Darcy!” [said Elizabeth] “Indeed I am. I shall entreat his pardon for not having done it earlier. I believe him to be Lady Catherine’s nephew. It will be in my power to assure him that her ladyship was quite well yesterday se’nnight.” [said Mr. Collins]
Elizabeth proceeds to attempt to dissuade Mr. Collins from introducing himself to Mr. Darcy to no avail. Mr. Collins does indeed introduce himself.
Why was Elizabeth so appalled at the notion of Mr. Collins introducing himself to Mr. Darcy?