10 Times Star Trek Broke Its Own Rules
4. The Romulan Cloaking Device - Minefield
Minefield is a second season episode of Enterprise that depicts the Romulans as having access to cloaking technology. This episode is set almost 100 years prior to the early episode of Star Trek, Balance of Terror.
In Balance of Terror, the cloaking device is depicted as being relatively new technology. Therefore, on-screen, Romulans seem to have a very different definition of what new and old are.
There is an explanation given for this, but it is not depicted in canon. Rather, the non-canon book The Good That Men Do explains that the Birds of Prey seen in Minefield were experimental ships, similar to the Klingon Bird of Prey in Star Trek VI. The experiments were deemed failures and it would take a long time for the Romulans to perfect the technology.
This is all well and good - but none of that is explained in the show!