Problem: Ace's craft can break reality... But it has been established that the crew of Red Dwarf (minus Lister) have been dead for 2 million years. When Ace arrived in this dimension, he would've arrived when the crew were still alive, or when Lister was kept in stasis to protect him from radiation.
Possible Answer: In the book "Backwards" by Rob Grant it says that Ace travelled 3 million years into the future because when his ship crosses reality it takes him to that reality's Rimmer at the same point in his life. So Ace is the same age as hologram Rimmer (Or effectively the same age considering when he died)
When Lister and Ace come back from fixing the engine, Lister looks dry, which couldn't happen if he just came in from a storm.
Kryten is not supposed to be waterproof, but he has gotten wet in Meltdown and Backwards.
Lister tells Ace that Rimmer is a hologram and "can't touch anything" yet almost immediately after, Ace asks Rimmer to help him fix the engine, which Rimmer cannot do.
Cat: Oh, it's bad, buddy, it's real bad. See what I mean? (Points at his own leg) Red with apricot. I look like a jerk. I'm bleeding an unfashionable colour. If I'd known I was going to get my leg crushed I'd've worn white. It goes with everything.
Lister: Is anything broken?
Cat: Yeah. All the stitching's come away, and the lining's ripped. Somebody, please, get me a tailor!
Lister: Kryten, get the First Aid box.We have to clean this up, make sure he doesn't get gangrene.
Cat: Gangrene? You think I might get gangrene?
Cat: Hey, that might work! Green with apricot, I think I could pull that off!
Ace: Smoke me a kipper, I'll be back for breakfast.
Holly: Purple alert! Purple alert!
Lister: What's a 'Purple Alert'?
Holly: Well, it's sort of like, not as bad as a red alert, but a bit worse than a blue alert. Kind of like a mauve alert, don't want to say 'mauve alert.'
Rimmer: Holly, wipe the rabid foam from your chin and start again.
Holly: There's some sort of disruption to the time-fabric continuum. At least, I presume that's what it is, it's certainly got all the signs. There's this big wibbly-wobbly swirly thing that's headed straight towards us.
Rimmer How would you feel if some git arrived from another dimension, another Lister, with a degree in bravery and a Ph.D in being handsome and wonderful?
Lister Hey, man, I am that Lister!
Mellie: If you're interested, I'll be in my quarters at lunchtime covered in maple syrup.
Ace: Sorry Mellie, I don't fraternize with the staff.
Mellie: I resign.
Ace: I'll be there at 1300 hours.
There is a real theory about diverging parallel universes called the Many-Worlds Interpretation of Quantum Mechanics. The writers no doubt knew about it.
Originally, the Cat's counterpart in Ace's dimension was to be a slovenly janitor. However, this was changed to the padre due to concerns that it would appear racist.
A different ending to this episode exists where Rimmer, after his net full of kippers fails to fall on Ace, stands underneath the net and it all falls on top of him. The full scene is included on the Series IV DVD.
The story of one Rimmer taking charge of his life and becoming a hero while the other one grows into the sad prat we know inspired a student to write into the offices of Grant Naylor. He had done badly in his mock exams, however watching this episode inspired him to try harder and ended up securing a University place as a result. The letter was framed and hung in Rob Grant and Doug Naylor's office as a reminder of just how rewarding their job can be sometimes.
The credits for this episode are a music-only version of the Red Dwarf theme on a simulation of a Hammond Organ, performed by Howard Goodall.
If you noticed that Mellie is the only one not to say "What a guy!" about Ace Rimmer, it was because a bit more of the maple syrup gag was cut from the final episode. The entire scene is included on the Series IV DVD.
Having crashlanded on an ocean world, Holly indicates that the damaged Starbug will begin to sink in about 40 minutes. Rimmer asks what they can do, and Holly suggests they could try and hire a band to play "Abide With Me"
An obvious reference to the sinking of the Titanic in the North Atlantic Ocean on April 14-15, 1912. To keep everyones spirits up, the band played on as the ship sank, and thus perished in the waters along with many others, given the shortage of lifeboats. "Abide With Me" is one of the songs reported as having been played by the band. The hymn they are alleged to have played at the last was "Nearer My God To Thee"
Visual: Space Corps Test Base/Mimas/Saturn
Mimas is the seventh of Saturn's moons, named after a Titan slain by Hercules in Greek Mythology. It was discovered by Herschel in 1789. In Ouroboros, we learn that Lister took planet leave there once.
Rimmer: He probably got to go to some really great school, while I was lumbered with Io House.
This, along with a line in 'Meltdown' is the only reference within the TV Series to Rimmer's birthplace. In the Red Dwarf novel Infinity Welcomes Careful Drivers, Rimmer states that his home is on Io, the third largest moon orbiting Jupiter. At 3630 km in diameter, it is slightly larger than Earth's moon. It was discovered by Galileo and Marius in 1610, and named after one of Zeus' lovers from Greek Mythology.
Rimmer: "Ace and Skipper"? You sound like a kid's TV series about a boy and his bush kangaroo!
Rimmer is referring to the 1970's Australian kids show Skippy which is, amazingly enough, about a boy named Sonny and his bush kangaroo, Skippy.
Film reference: Top Gun
The "heroic fighter pilot" storyline and music is reminiscent of the 1986 movie Top Gun, starring Tom Cruise.