During filming, John Kapelos told Emilio Estevez and Judd Nelson that they should be more dedicated actors, joking that they would've let Martin Sheen die if they'd been on the set of Apocalypse, Now (Sheen had a heart attack on the set). Kapelos didn't realize that Sheen was actually Estevez's dad and felt really embarrassed when he learned this. (Source)

Judd Nelson was almost fired after he continued to remain in character and harass Molly Ringwald when they weren't filming. The other actors talked Hughes out of it. (Source)

Rick Moranis was originally going to play Carl, but Hughes and producer Ned Tanen fired him after Moranis insisted on wearing a Russian hat and doing a Russian accent for the role. (Source)

Molly Ringwald and Anthony Michael Hall actually dated for a while when they were making the movie. So, Hall actually did get with someone—in real life, if not in the movie. (Source)

When they arrived at the school where they were going to film, the actual school library was too small. So the crew constructed a larger library-set in the gym. (Source)