Revealing mistake: When the [CGI] Titanic leaves the dock, watch a man in a green jacket close to the bottom part of the screen. He is waving goodbye to 'nowhere', for that part of the boat is empty and its prow is pointing away from where he is staring. This reveals he was looking at a green screen, with no idea whatsoever of what was going on. (00:25:40)
Continuity mistake: Ruth walks to the ship, followed by Rose and Cal. When she is about to enter, the angle cuts and suddenly several women with dogs haven taken her place, followed by a lady who looks nothing like Ruth. Then in the following close-angle the group is meters behind from where they first were, repeating the walk towards the door. (00:22:15)
Factual error: The movie shows men removing the boats that have been stowed (Collapsibles A - D). The boats that they remove are not collapsibles, which were wooden bottom boats with rolled down collapsible sides (hence the name). (02:18:10)
Factual error: Shortly after Old Rose mentions just leaving the coast of Ireland, a shot of the captain's area (with the steering wheels) is shown with the sun shining in from the right (if facing toward the front of the ship) This would be impossible for a westbound ship in the middle of the day as the sunshine could not enter from the north side of the ship. The rest of the movie has all scenes with the sun correctly coming from the south during mid-day as the ship is travelling westbound.
Factual error: When Rose is trying to rescue Jack, the corridors that she is running through are a bright white colour, the lights appear to have a high colour temperature, and they have a more modern A19 bulb shape, and they might be halogen. In reality they would emit a much dimmer yellowish light. The lightbulbs would also be transparent along with a different bulb shape.
Continuity mistake: When Lovejoy and Jack are talking, Jack tucks his cigarette in his ear in an 11 o'clock position. A shot later it's in a 9 o'clock position and a bit of hair is covering his ear.
Other mistake: The ship is down by the bow about 30°. There follow scenes in which people make their way through flooded passageways. The water is chest-high, but level.
Continuity mistake: On the night of Rose's attempted suicide, we see her with a necklace on at the meal which disappears when she reaches the end of the ship. There was a deleted scene showing her removing it in her room, but without that it just vanishes. (00:35:00)
Revealing mistake: When Jack spreads the handcuffs over the pipe, we have a very clear close up shot of the links connecting the cuffs. In this shot you can see that the link nearest Jack's right hand is broken and not a complete link, therefore he should be able to separate the cuffs without going through the axe sequence. (02:00:05)
Continuity mistake: The way the necklace is held after it is found in Jack's pocket changes instantly. In one shot the steward is gripping the chain. In the next, the chain is draped over his fingers. (01:41:10)
Visible crew/equipment: When Jack and Rose are running from Lovejoy, they stop for a moment on E deck. When they do, look behind them and a member of the ship's crew comes into shot holding a bag of something. When Jack and Rose start running again, this man has disappeared. (01:28:20)
Continuity mistake: In the film, Titanic has a yellow stripe down from the bow to the stern, however the yellow stripe disappears from the film in some scenes, and appears in others. The main one is the sinking. (01:59:55)
Continuity mistake: Just after Rose leaves Cal, we get an underwater shot panning down on Jack, showing that the window he's standing next to is fully submerged. However, interior shots after that show air around the upper edge of it. It's not trapped air: it's constantly bouncing around, in a wave-like pattern. (01:53:10)
Revealing mistake: When Tommy and Fabrizio meet Jack and Rose at the bottom of the staircase and try to look for a way out, you can see a boy in the bottom left corner staring at the camera and smiling. (02:03:40)
Continuity mistake: In the "You jump, I jump" scene with Jack and Cal persuading Rose to get on a lifeboat, Rose embarks the lifeboat at the front, port(left side) side of deck top. Yet when Rose jumps back on the ship, she climbs back on in the middle, starboard(right side) of A deck where the windows change, running towards the bow to the Grand Staircase; while Jack is running towards the Stern from the Port front side of deck top. Jack enters the Grand Staircase from the port side of Deck top, and Rose enters the Grand Staircase from the port side of A Deck. (02:11:40)
Continuity mistake: When Rose leaves Jack's company to retrieve help, and goes to return with the axe, she stops at the stairway with the rising water and wedges the axe on the bars. The camera cuts to behind Rose to show her removing her outerwear, and the axe is still being held up by the bars but it's nice and straight, before and after this camera shot, the axe is propped in the bars crooked at an angle. (02:00:05)
Revealing mistake: In the first shot of a sailor watching the ship right in front of the iceberg, the mast has thick blue lines around it and the man's coat and cap swap from black/dark blue to faded blueish colour, all due to the green screen. (01:34:45)
Continuity mistake: Rose steps out of the car, sees the Titanic and says "It's as big as the Mauretania" while she holds her hat. A frame later, from a different angle, her arm is down. (00:21:00)
Continuity mistake: At the beginning of the film when Cal sees Rose putting her paintings up, if you watch closely in the background, in one shot Cal walks into the room with his glass at his side, in the following shot it is at stomach level. (00:27:50)
Revealing mistake: When the cop find Rose's necklace in the coat, Jack says "he put it in my pocket" and Cal tells him to shut up. Right then, he turns his head and the glue of his wig is noticeable around his temple. (01:41:30)
Chosen answer: During the several years it took to construct the ship probably, or in any of the supplies/food brought on board, or in the furniture brought on board. A single pregnant female rat can be responsible for thousands of rats in a very short space of time (the offspring are not too choosy about who they breed with).
Soylent Purple
A pregnant female rat could have made a home in a underneath a third class couch and had the other rats then all the females would have baby rats quickly.