Revealing mistake: Notice the land in the background of the ship which is supposed to be in the middle of the North Atlantic. It is in one of the whole shots of the boat which has Brock and his team on it. (00:08:30)
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)
Revealing mistake: When we see the dogs being led on to the ship at the very start, look behind them and there is a woman on the walkway waiting for her cue to start walking. (00:22:10)
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)
Revealing mistake: As the Titanic is leaving you can see the news guy holding a hand cranked camera. He's cranking it left-handed, but all hand-cranked cameras are right handed. Either a construction mistake or revealing that the shot was flipped - a mistake either way. (00:25:50)
Revealing mistake: Many scenes used computer graphics to show the length of the ship. Passengers were also added walking on deck. The shadows for the passengers don't always match. (00:30:25)
Revealing mistake: During the first meal on board, the make-up glue around Cal's hair-piece is very noticeable. (00:32:15)
Revealing mistake: In the suicide attempt scene, after Rose climbs over the railing, you can notice banding of the night sky a couple of times - that appears on both VCR and DVD. (00:37:15)
Revealing mistake: When Rose is about to commit suicide and Jack approaches, their skin turns pale and slightly blueish due to the green screen effect. This happens in all the wide angles from this scene where a big part of the sea and the horizon are visible. (00:38:30)
Revealing mistake: When Jack rescues Rose from her suicide attempt she slips. In the shot from behind when she is seen dangling precariously her (or the stuntwoman's) hair is longer and redder. Kate's stunt double is noticeable in other shots throughout the film. (00:40:55)
Revealing mistake: If you look at the deck planks to the right of Rose and Jack you can see that there is something that runs perpendicular to the rest of the planks. This wasn't on the real Titanic. This is where the set was "cut" so that the front half could be sunk into the tank later on in the film. (00:51:00)
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)
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)
Revealing mistake: When Rose is running in the hallways trying to find help for Jack when he is hand-cuffed, she finds a man and asks him to help her free Jack. She gets frustrated with him and says "listen" then hits him. In shots before you can look at the man's hand and it already has blood on it before he touches his face. This was fixed in Blu-ray version. (01:59:05)
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)
Revealing mistake: When Rose breaks Jack's handcuffs with an axe, it is obvious that she doesn't hit the handcuffs on the pipe but Jack's hand - the handcuffs come apart by themselves. The axe also bends. (02:01:30)
Revealing mistake: During one side shot of the ship sinking at night, the bow has clearly begun to slide under. There are several lines hanging off the side of the boat. The lines are perpendicular to the boat, not the water, indicating that the model was filmed when level and tilted digitally. (02:02:25)
Revealing mistake: You need to watch carefully for this as it's tricky to spot, but it is visible without the need of slow motion. When the steward calls for the gate to be unlocked to the third class passengers, the gate is already open and just pushed to a close. You can see the gate wobbling and separating around the lock area. Also as the gate is unlocked we see a key being pressed against the lock to unlock it. However the key doesn't actually engage with the lock, making it impossible to open it. (02:02:50)
Revealing mistake: When Tommy, Jack and Rose are trapped down the staircase with a lot of other people, during the shot where Fabrizio is running towards Jack, look on the right portion of the screen. You can see a kid with a hat on, being carried around by his mother, who deliberately grins at the camera. (02:03:40)
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)
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.