Enterprise introduced cloaking technology a century before "Balance of Terror". "The Enterprise Incident" then reintroduced cloaking technology; Spock and Kirk (wrongly) act as if it is a brand new development despite both actually being in the episode where they previously (wrongly) acted as if it is a brand new development.
It's that damn 23rd century beaming technology, scrambling their memories. BONES WAS RIGHT ALL ALONG.
It's that damn 23rd century beaming technology, scrambling their memories. BONES WAS RIGHT ALL ALONG.