Quick, where were you when you heard about the 9/11 attacks? Who were you with? If you’re like most people, you’ve got answers at the ready. But as it turns out, there’s a decent chance your answers are wrong, reports Time. That’s the upshot of a new study in the Journal of Experimental Psychology, which examines people’s remembrances after the attacks. In the days after Sept. 11, 2001, researchers across the country asked more than 2,000 people about their personal experiences with 9/11. When researchers followed up at later intervals, about 40% of the stories—or “flashbulb memories,” as they’re called in the vernacular—had changed. “Human memory is not like a computer,” says study author William Hirst of the New School for Social Research. “Human memory is extremely fallible.”