Yes!  In the first episode the Defiant appears, Sisko explains that the Defiant prototype was 'mothballed' when the Borg threat waxed then waned; perhaps they kept the registry as is as a temp measure or to obfuscate viewers.

As well, supposedly, while on training exercises, the Valiant with crew and cadets, was somehow disabled (attacked, I think), command and other officers/crew were killed, and cadets were then in charge (but didn't know how to repair the warp engines sufficiently).  (Big mistake.) )