Lawrence O'Donnell has a good summary of the recent gaffe.
To me, there are only three explanations:
1) Her staff really is that stupid and the genesis of the Lexington and Concord screw-up can easily be explained. They knew she was going to New Hampshire, so they did some googling and saw that one of its largest cities is Concord. A quick trip back to Google took them immediately to the Wikipedia page on Concord and the rest is simple plagiarism.
2) Someone on her staff intentionally inserted that statement hoping to intentionally sabotage her. This appears unlikely, since we have not been informed that any staffers need to spend more time with their family.
3) Someone on her staff intentionally inserted that statement on a dare, to prove to fellow staffers that they really can get away with anything, and the 'trusted' media will not make an issue of their humorous little prank.
I guess my vote is still #1. There really are bozos on that bus.