I have several names that come to my mind:

-The 1945 Luftwaffe Kamikaze on the east front(only delayed the war for 24 hours and costs many lives + aircrafts )
-The Mistel (one airrcraft put on top of the fuslelage of another)
-The Maginot line; it was hardly used, cost a fortune and the spare dmoney could have build hundreds of aircrafts, tanks and change a lot of things
-The Biber submarine which turned its user into a manned torpedo with very little chance to survive.

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