Harry Potter and The Half-Blood Prince - Slughorn Refers to Ron as Rupert
On page 485 Prof. Slughorn says "Had a house-elf taste every bottle after what happened to you poor friend Rupert." Rupert is the name of the actor who plays Ron in the Harry Potter movies.
Slughorn doesn't remember Rons name at all and calls him all sorts of names that aren't his. Ron isn't important enough to Slughorn to remember his name.