Rupert

From UnknowableWiki

Rupert is a nickname used by Horace Slughorn to refer to Ron Weasley in a failed attempt of the professor to remember his pupil's name (HBP22).

Initially, many fans thought J.K. Rowling had made a mistake, because she would've been thinking of Rupert Grint when she wrote the words. However, it was later confirmed to be intentional.