I was on a study abroad programme from my American university. During an Italian lesson, our professor burst into the room and shouted that the Twin Towers had been attacked.
I was quite an immature idiot and didn't immediately understand the implications of what had happened. All I could think is that everything is now unsafe and how could I get in a plane again.
All of us students went out, wandering the streets in shock, and found ourselves in a hotel lobby where they were showing the footage.
I watched the US transform from afar during that term. When I came home, I was totally weirded out to see patriotic Christmas lights for the first time ever.