In the beginning of the season 3 TOS episode "The Enterprise Incident", the Enterprise crew are taken off guard by the sudden appearance of two additional Romulan ships. When Kirk asks Spock, he speculates that the Romulans must have developed a cloaking technology.
But they already faced a cloaked Romulan ship in the season 1 episode "Balance of Terror".
Is this just a continuity error or is there an explanation for this?
From The Enterprise Incident's Memory Alpha Continuity section:
The Romulans have improved their cloaking system since "Balance of Terror" and "The Deadly Years". In the former, Spock refers to it as an "invisibility screen" that can mask a Bird-of-Prey's appearance, but not its motion. In "The Enterprise Incident," the cloaking device now renders a ship undetectable while moving, and not even the Romulans themselves can track a vessel so equipped.
They're surprised because they didn't detect the ships approaching, due to the Romulans' new and improved cloaking technology.
Or possibly another explanation might be that "The Enterprise Incident" and "Balance of terror" happen in different alternate universes and in each episode it is the very first appearance of a Romulan cloaking device. Thus Kirk and Spock would be equally surprised in both episodes.
