USS Excalibur
From FEInfobase
First Excalibur Class Starship, lost during its maiden voyage. For many years it was believed that the ship suffered massive damage from it's quantum slipstream drive malfunctioning, then was inevitably destroyed by a Borg sphere about 63 light years from the Sol System.
However in 2403 the more recent USS Excalibur, was sent on a mission by Rear Admiral Donatra to investigate strange energy readings that some of her ships in the Rapid Response Task Force had picked up. When the Excalibur arrived she quickly found her predecessor. After a few hours of rejoicing and finding out what happened from the other Excalibur's crew, the old Excalibur's captain, Captain Dimitri Alexandrovich, stole several engine systems and parts from the new Excalibur's cargo bay.
It was soon realized that the man had gone mad and his actions were unrationalized by being stuck out in open space for ten years with no way to go anywhere. Due to some redesigning of the quantum slipstream drive systems since the first Excalibur's launch, the engine parts weren't compatable. When the first Excalibur went to activate her slipstream drive the ship was destroyed. However the current Excalibur was able to transport over 520 out of 723 of their crew.
The entire matter was unanimously agreed to be classified by Starfleet Command and the first Excalibur Class USS Excalibur is still officially considered missing in action.