When it comes to weapons failure, the panjandrum, or "The Great Panjandrum," is right near the top. Designed by the British during WWII, it was basically two wheels held together by a bomb and included rocket propulsion. The plan was to break out the panjandrum on D-Day in order to penetrate the Nazi's coastal defense and fortifications, the Atlantic Wall.
The Panjandrum was projected to break through the concrete Atlantic Wall in order to create a gap wide enough for tanks to penetrate. The theory seemed flawless and the weapon was thought to be feasible and an effective. It would allow the British to storm the beach without sending a large numbers of soldiers to face lethal enemy fire. However, a plan and execution are two very different things in war.
However, after a few modifications, the panjandrum proved during its final test to have the reliability of a last-minute high school mouse-trap car project. The result: one dead dog and one dead project.
As the rockets continued to fire the Panjandrum spun erratically, and people (generals included) went running for cover. The cameraman was almost bowled over, and eventually the machine disintegrated. The dog of one of the army officers present chased down one of the rockets strewn on the beach and was killed by it.
Head over to War History Online to read more and check out videos of the failed test and a recreation of the prototype from 2009 on the 65th anniversary of the Normandy landings.