Deliberate mistake: When Rojan asks Kelinda if she wishes to remain with Kirk, she replies that although Kirk is "most interesting" she wishes to go with him, and the next closeup of Rojan is flipped. (00:48:15)
Continuity mistake: When Scotty gets drunk and tosses an empty bottle away, there's a dubbed in sound of glass breaking, but when he collapses, the bottle is unbroken at his feet.
Chosen answer: If they're in orbit, they're being pulled along by the planet's gravity well, therefore, impulse engines would only be used for minor corrections and would be "on standby" while in orbit, but not active. (Like keeping your car idling without revving the engine and creating plumes of exhaust).
Captain Defenestrator
Thank you for the info.
Movie Nut