Revealing mistake: When Cal steps out of the car just after the beginning of the movie, you can see behind him the back of the Titanic replica still incomplete, with the internal structure totally visible. (00:22:06)
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.
Continuity mistake: When Jack and Rose are in the lower hallway after escaping from Cal, Titanic is already at a major tilt with the bow completely underwater and the first class dining saloon getting flooded. The problem is that the water in the hallway is completely level and the water that floods is also level. With the ship tilted the way it is the water should have been collecting at the end of one of the hallways, not flat.
Continuity mistake: The first chimney fell in the scene where Fabrizio died, but later when showing the Titanic sinking, the uppermost part of the chimney is visible, intact. (02:40:30 - 02:41:15)
Revealing mistake: When Jack and Rose are talking at the bow of the ship before the "I'm flying" part, if you look at the windows behind them you can tell it is a painted prop area and not a real ship.
Revealing mistake: When the boat is totally slanted sideways and Rose encounters Andrews, note the glasses and stuff over the fireplace and behind Rose. They don't slide downwards or move a single inch, defying the law of gravity, revealing the camera trick. (02:21:00)
Revealing mistake: Just after Rose spits in Cal's face and runs off to find Jack, the camera goes into the water to show Jack through one of the portholes. In the external shot, it shows that the porthole is a good distance under the water. However, just as Jack says "Hello? Help me" you can see the water dip below the top of the pothole a number of times as it sloshes about.
Factual error: The ribbons on Captain Smith's medals are incorrect. The ribbon on his Reserve Decoration should be plain green (it is green and red in the film) and that on his Transport Medal should be red and blue (it is blue and white in the film).
Continuity mistake: When Jack is manacled in the master at arms' cabin as it floods, the drawers from the left-hand side of the desk behind him are floating around in the water, then for a short shot are back in the desk and then floating around in the water again.
Continuity mistake: When Rose is hanging off the back of ship she first isn't holding the train of her gown then suddenly she is.
Visible crew/equipment: When old Rose is traveling in the helicopter at the beginning of the movie, the skids of the filming helicopter can be seen reflected in the window of the one Rose is in. (00:13:02)
Factual error: In the seen after the preacher finishes his dialogue; it shows the woman floating in the first class lounge with which is completely under water. But about a minute or so later after it shows the maid slide down the deck, the part of lounge that was supposed to be flooded is still above water. The lounge in just a bit forward of the 3rd funnel on A Deck which wasn't submerged until after the break up in the film. The light fixture is one deck down aft of the compass tower between funnel 2 and 3.
Factual error: When Rose's mother is having tea, all the women are wearing gloves as they have tea. This would never have happened as Edwardian ladies always removed their gloves and placed them in their laps under their napkins when they sat at a table, before eating or drinking anything so as not to soil them.
Continuity mistake: When Jack looks through the porthole (which is full submerged underwater) while cuffed, he looks over his left shoulder which is facing the hull of the ship. He then turns his head which cuts to him inside the room, his left shoulder is now facing away from the hull of the ship, towards the door. The shot of him looking through the porthole is possibly a reversed shot.
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.