The first ending was axed

The first screening of the movie went so brutally wrong that people actually started booing the end, forcing John Hughes pen another finale.

Susannah Gora wrote in her book ‘You Couldn't Ignore Me If You Tried: The Brat Pack, John Hughes, and Their Impact on a Generation’: “A test screening of teenagers who were watching and loving 'Pretty In Pink' actually booed when they saw the ending where Andie and Duckie end up together, after a lot of soul-searching, the filmmakers ultimately decided to give the audience what they wanted: Andie and Blane ending up together.”