The Space Shuttle Challenger explosion ... probably everybody who was alive back then remembers where they were. I remember explicitly - I was in the lobby hanging with some buddies during lunch. My homeroom teacher burst out of the teachers lounge screaming and crying. We had no idea what the problem was - we thought it was a joke at first.
When we got back to the room, we spent the rest of the afternoon watching TV -watching the updates, and trying to figure out what happened and why. I think we all lost some innocence that day (we thought nothing could go wrong with the space shuttle and space exploration ... heck, there'd been so many shuttle launches, that we became numb to it).