Star Trek IV: The Voyage Home (1986)

Suggested correction: According to the captain's log at the beginning, they have been on Vulcan for 3 months. As they prepare to depart, we see several Vulcan technicians moving equipment in and around the ship. It's quite conceivable that the bridge was reconfigured according to the crew's specifications to facilitate their use of the ship. This may seem a bit excessive, as the remodel includes the door onto the bridge, and the frame of the door, and possibly most of the rest of the ship. But it's not outside the realm of possibility. The real mistake, though, is why they would go to the effort of installing new workstations on the Klingon bridge and marking them with Klingon labels, instead of standard Federation text. Interestingly, though, the "Starfleet" style bridge layout of the Klingon ship is being used by the Klingons on the Bird of Prey in Star Trek V.

Vader47000

Chosen answer: This question has been answered a number of times by various individuals, all saying pretty much the same thing. The answers have been most satisfactory given the question revolves around a fictitious situation and the answer (s) need to be accepted as complete for this purpose. Any dispute or non-acceptance should be addressed in a Star Trek forum. Any ignoring of the Prime Directive was done to save the future of Earth, as the probe would have wiped out all life on Earth. Essentially, nothing that was done in the past resulted in major changes that would make Earth 300 years later appear any different, and no major futuristic technologies were revealed. The major one, Chekov's communicator and phaser being left behind did not result in anybody learning secrets. In the film, the phaser didn't function because of the radiation. It's presumed then the radiation permanently damaged the equipment so it appeared to be nothing but a toy or prop. However, in the novel "The Eugenics Wars: The Rise and Fall of Khan Noonien Singh", Roberta Lincoln was sent by Gary Seven to recover the items from Area 51 before any secrets were learned (and as stated before, additional corrections to Earth's timeline could have been done that aren't addressed in the film.) The subsequent loss of a suspicious "ruskie" would have hardly affected the era that was already in the midst of the Cold War. McCoy even questions that giving Dr. Nichols the formula for transparent aluminum could alter history to which Scotty replies what if Dr. Nichols is the one who invents it, to which McCoy agrees (in a later novel it is reveled that Scotty already knew Dr. Nichols invented transparent aluminum, so history was not changed.) The miraculous recovery of the old lady (growing a new kidney) was done by a pill so that any examination of her would not reveal the futuristic method involved. She would be a bewilderment to the medical community at best, and most likely misdiagnosis would be to blame. And just because she got a new kidney does not mean her life would have been extended, she could have died some other way in both timelines. And as stated before, Gillian simply wasn't vital to Earth's history. She could have contributed nothing of importance to society and died alone and childless. And a missing pair of breeding Humpbacks would hardly affect the extinction of their species, however in the future, they are already extinct, so little changes would occur. As for any questions about people seeing the Klingon ship in the past, who would believe them? People have long been claiming to see spaceships and aliens to little or no avail, so why would anyone believe a handful of people who said they saw aliens in a spaceship steal 2 whales? However, as with many time travel situations in films and novels, it's possible the events of the 23rd century as they appear in the beginning of the film are a result of Kirk and company's actions in the 20th century since the events already occurred even though Kirk and company had not yet done it themselves (this is where a discussion forum on the film would be advised, or a discussion forum on the theories of time travel).

Answer: They were extraordinarily lucky. The crew quite often defies all odds and encounters literal miracles. For a period of time this even happened on a roughly weekly basis.

TonyPH