On this day in 1986, the Space Shuttle Challenger exploded over the Atlantic Ocean:
A little more than a minute after launch and high above Kennedy Space Center, shuttle Challenger was ripped apart after failure of a rubber seal allowed a spurt of rocket flame to ignite the spacecraft's giant fuel tank.
The roiling plume of Challenger's disintegration would sear an image in the nation's psyche that spoke of a particular sorrow; among seven astronauts killed 30 years ago [today] was teacher Christa McAuliffe.
"We will never forget them," said President Ronald Reagan in a broadcast hours later evoking triumph from tragedy. "The Challenger crew was pulling us into the future, and we'll continue to follow them."
Studying the decisions that led to Challenger launching that day uncovered serious cultural problems within NASA. It also led to groupthink about groupthink as people continually mis-applied the lessons from the disaster after not actually understanding them.
I remember exactly when I heard about the disaster: I was taking a final exam in high school when the PA broke in to tell everyone it had happened. (We still had to finish the exam.)