Archaeologists make mistakes. I recently ran across this crown-that-was-actually-a-bucket story as well.
In a bit of serendipity, the curators realised during research for the show that an object they had long assumed was a vase had actually been displayed upside down. They now understand that it is actually the head of a fired-clay mace, or heavy club, made for King Gishakidu of Umma. After comparing the object with a similar one at Yale University, "we realised how daft we'd been", says Irving Finkel, a co-curator of the show. Now displayed right-side up, the mace head is topped by a painted representation of a net that was used to immobilise enemies for execution.