8. Which Ship Was Studying The Gas?
In the opening scene of Star Trek VI: The Undiscovered County, Captain Sulu records his log aboard the USS Excelsior. He states that the ship is en route back to Starfleet HQ, having completed a three-year mission cataloguing gaseous anomalies in the Beta Quadrant. Moments later, the ship monitors the explosion from the Klingon moon of Praxis, kickstarting the action in the film.
In the climactic battle between the Enterprise-A and Chang's cloaked Bird-of-Prey, Uhura solves the riddle of how to locate the enemy ship. She suggests using the extra material on-board - that they had been using to catalogue all of those gaseous anomalies - to find the ship's tailpipe. This seems to have been a case of Checkov's gun moving from ship to ship.
According to George Takei, this change - the Enterprise saving itself, as opposed to the Excelsior saving the day - was made at the request of William Shatner. Takei claims that Shatner believed 'no one but Captain Kirk can save the Enterprise.' The scene, whoever was responsible for saving the ships, is a thrilling one in any case. Just don't ask which crew is supposed to report on the Klingon plasma emissions.