Here's a nice grading puzzle, smaller versions of which sometimes come up in law school exams: How should you deal with students who make mistakes about basic facts that are not directly relevant to the question:
I have now received three (3) student papers that discuss Iraq's attack on the Twin Towers on 9/11. All three papers mention it as an aside to another point. I've had two papers on the virtue of forgiveness that argue that if we had just forgiven Iraq for the 9/11 attacks, we wouldn't be at war right now. I just read a paper on the problem of evil which asked why God allowed “the Iraq's” to attack us on 9/11.
The thing that upsets me most here is that the the students don't just believe that that Iraq was behind 9/11. This is a big fact in their minds, that leaps out at them, whenever they think about the state of the world.