I think the mistake must reside partly with the Bush administration for using the term "mission accomplished." I think to most people take that to mean the hard part is over, and that's why a time table to finish cleaning up is expected.
While it was the accomplishment of a major objective, it didn't marke the completion of the Iraq mission. Call it a mistake based on initially underestimating the insurgency, but it became apparent shortly after the "mission accomplished" announcement that this was not the case, and probably should have been more forcefully retracted, until the overall mission - which I take to be an independent, democratic, stable Iraq, free of Saddam Hussein, with the ability to defend itself against both internal and external threats - is completed.