Mohamed Atta's luggage didn't make it on to Flight 11, and as a result a number of clues about the attacks were left behind for the FBI. It's been said this was suspicious, a trail left for the FBI to follow (although arguably the hijackers would have been identified even without it), so why was his luggage left behind?
This does seem to be something of a mystery, or at least not an issue that's been covered extensively online. Here's the one article we've found that offers a partial explanation.
This isn't entirely satisfactory, however, as it relies on an anonymous source, and only pushes the cause back one stage. (That is, we're told US Airways were late transferring the bags, but we still don't know why.) We'll add more to this page as and when we discover additional clues.