Searching for an old story about an officer put in command of a museum ship to fight in an important space battle against an enemy with a surprising new weapon. He wins, because it turns out the new weapon is an old weapon redeveloped which the museum ship had a defense for.
Possibly The Ghost Fleet by Christopher Anvil.
The protagonist is forced to use a museum ship in battle; it has a weapon called the "disrupter" mounted in it that was used in the past. Countermeasures were soon devised for the disrupter, so use of the weapon was dropped, and both it and the countermeasures were forgotten. So when the protagonist uses the museum ship, the enemy ships no longer have the countermeasures.
Beller handed the General a small placard, such as is used in museums to explain exhibits:
"The disrupter countervail was used during the period of the Early Space Wars to counteract by split-phased resonance the action of the nuclear disrupter. The nuclear disrupter was in this period used to unbalance the binding forces of the atoms, causing a rapid collapse of the structure of the attacked ship. The disrupter, however, required the use of such large and complex equipment, and its action was so easily countered, that it soon fell out of use. For many generations, the countervail was carried on ships as a matter of routine precaution, though the disrupter itself has seldom ever been used since the early space wars."—Exhibit C, Science and Industry.
The General looked up.
"Orders from the capital removed all remaining countervails on the older ships several years ago, to save space and weight. No one raised a question. We'd never had any use for the things. When the trouble you mentioned started, first it fit the standard pattern of... excuse me... a frightened man making an alibi for himself, and then it looked as if it might possibly fit the standard pattern of a brand-new weapon. It never occurred to us it might be an old weapon revived. We were victimized by too much reliance on standard patterns."
The story is available online here (legitimacy unknown)