Not sure I agree with this. If something appears in a episode, it's best to figure out a way to wiggle it into continuity somehow. If necessary I would rather put somethings down to a mis-statement on the part of a character, than "retcon" it out of existence.
Some fans want to retcon way too much out of the show, simply for the purpose of supporting their own position on a personal viewpoint.
No I agree. It some thing that should only be used when the writters have made a really really really dumb mistake. Like implying something completly impossible or really stupid.
There are certain episodes were writers have droped the ball so badly like threshold and These are the voyages ect
As for Romulans have only sublight speed. It is impossible end of. Even if that one ship only had litral impulse it would have to have been carried there by a warp ship. If they did not have faster than light they would never have left there system let alone been able to wage a instersteller war with Earth, Vulcan, Andoria and what ever the tellerite home system is called.