I hit a nasty pothole in Boston last week. The CrossRack bag went flying (wasn't strapped quite tightly enough). When I got home, I noticed that I had also broken I spoke. I suppose it's possible the spoke broke later, but my guess is it happened at the pothole ... which was 6 or 7 miles from home.
The amazing thing: when I took it to the LBS today to get a replacement spoke, they said that pretty much all the other spokes still looked true. I'm amazed because every time I've broken a spoke on other bikes, the wheel has gone way out of alignment.
That's one strong wheel!