A Constitution-class starship whose entire crew was killed when the M-5 computer turned a war game into real combat.
The USS Excalibur was a Constitution-class starship of the 2260s, commanded by Captain Harris. In 2268 it was one of the ships assigned to a war-games exercise testing Richard Daystrom's M-5 multitronic computer, which had been installed aboard the Enterprise.
When the M-5 mistook the exercise for real combat, it opened fire on the other ships with full-strength phasers. The Excalibur was crippled and its entire crew was killed, alongside heavy losses aboard the Lexington, before Kirk persuaded the computer that it had committed murder and it shut itself down.