Revealing mistake: When Jack and Rose are running from the raging water that is gushing down the hall after them you can see the faces of the stunt doubles through the computer generated ones of Leonardo and Kate (slow motion can help but isn't essential). (02:17:15)
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: 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 Rose lets go of Jack's hand and he sinks to the bottom, if you look closely enough, you can see Jack's eyes move under his lids, even though he is supposed to be dead already.
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 the ship tilts to a vertical position, you see people falling and hitting objects on the deck. In a close up of that, you can see that a black and bronze-topped capstan has wrinkles in it and bends when someone hits it. Fixed in Blu-Ray version. (02:36:40)
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: 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 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: As the ship is breaking apart, you can see the wires that pull people into the tear on the deck. (02:40:10)
Revealing mistake: When Cal shoots at Rose and Jack he hits a decorative wooden structure on top of a banister. When the piece falls apart you can see that it has been neatly cut in the middle, and you also see the explosive device that blew it apart. (02:14:40)
Revealing mistake: This mistake, as far as I know, can only be seen in widescreen. After Rose, Jack, Tommy and Fabrizio have run on deck (after breaking the gate down), the camera looks down the ship toward the bow. If you look at this carefully, you can see city lights in the top-left hand corner of the picture. I know these are city lights because in a book about the movie, it shows a similar picture. Fixed on Blu-ray. (02:08:05)
Revealing mistake: When the ship is sinking, there is a shot of Victor Garber leaning against the wall in deep contemplation. The camera angle is very tilted and a few things slide off the shelf in front of him. Look carefully on the DVD and you'll see a wire coming out under his arm behind him pulling the china cup off the shelf. Fixed on Blu-ray. (02:25:25)
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: In one of the sinking scenes, you can see the rectangular strobe lamps showing through the black fabric in the glass dome. (02:30:35)
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: Just after the Titanic has sunk, and all the people are thrashing around in the water; look towards the top of the screen as the camera pulls back, and you can see the back of the tank it was filmed in. It shows as a clear line against the background. (02:37:40)
Revealing mistake: When Rose and Jack first enter the boiler room, they run towards the right. If you look closely you can see the reflection of the worker in the back to the left in the mirror they use to make the bolier room look longer. In the next scene they are running in the opposite direction, because the set is so small due to the mirror, and once again you can see the same reflections caused by the mirror.
Revealing mistake: In the engine room as the ship is sinking, the smoke and steam coming from the machinery rises at the same angle as the ship, about 45 degrees. So, the shot was filmed level, then tilted afterwards. (02:40:40)
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)
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.