#4 in the memories we'd rather not have of Bush...September 11th.
The root causes of 9/11 go too far back for it to be all Bush's fault, clearly. However, there are some things he and his administration missed and some things they could have done afterward. First of all, there was the Presidential Daily Briefing (PDB) that said (in fact it was titled) Bin Laden Determined to Strike in U.S. from August of 2001.
More damning, I think, is the response. Bush was not quick to act. He stayed in a classroom reading to children, then he flew around in Air Force One for a while before returning to Washington D.C. and speaking on television. Action against al-Qaeda in Afghanistan was necessary but military action may not have been the best way to go. Some have suggested that a smaller scale police action may have been more effectve in capturing the heavies in al-Qaeda. In any case, Bin Laden is still somewhere out there.
There are more issues that arise out of the subsequent "War on Terror" (hello torture, Iraq, etc.) that just my appear soon in this countdown.