Continuity mistake: When Jack and Rose are going down with the ship, there is a man holding onto the flagpole. The man's life jacket disappears and reappears. (02:42:42)
Visible crew/equipment: When Jack comes to the first class door for the first time in his tux, you can see a cameraman in the glass door just before he enters. (00:54:55)
Deliberate mistake: In the scene at the start of the movie with the "authentic" piece of film, you can see a girl with a hat waving to the people at the ground. When the ship is leaving port in the film, she and others are in the same positions, only in reverse, because the first "authentic" shot is the same shot, only mirrored. (00:00:40 - 00:25:30)
Continuity mistake: When Rose breaks Jack's handcuffs, you can see the stunt person not wearing suspenders as Jack does. This is an inter cut shot between the raising of the axe and the striking of the cuffs. Leonardo's stunt double is noticeable in other shots throughout the film. (02:01:00)
Continuity mistake: Just as Jack begins to draw naked Rose, he draws a dark line down the center of his paper. In the next shot of the page the line is suddenly much fainter (over to the right) and he is now drawing her face. (01:23:15)
Continuity mistake: When Rose is trying to rescue Jack she spies a fire axe. Smashing all the glass out from the holder she grabs the axe and turns round. The next camera shot shows Rose standing in front of the case with almost all of its glass intact. (01:59:25)
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)
Continuity mistake: Look closely at the location of Rose's beauty-mark the first time you see her at the dock. It is on the opposite side of her face during the rest of the movie. (00:21:30)
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 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)
Continuity mistake: Just as Titanic breaks apart, the compass platform (seen in front of the third funnel when the lights were still on) suddenly disappears. The water isn't even high enough to have reached its aft-most legs. The white projection on the deck that held the chandelier in the lounge is also gone, and the ventilator that sits on it is flush with the deck. (02:40:30 - 02:41:40)
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)
Continuity mistake: When Jack and Fabrizio are at the bow, the anchor well below them is black. Along with that, the foremost railing is not connected with the rest. But when Rose and Jack are there the well is white and the railing connects. Plus during the "flying" sequence the gap between the vertical bits of the railing is different in different shots of the ship. (00:30:40 - 01:17:35)
Visible crew/equipment: When Jack and Rose are running away from Cal to the first class dining room, if you look at the glass you can see a black screen, a light, and a crewman. Fixed in the Blu-ray version. (02:20:32)
Continuity mistake: When the ship is about to leave the dock, there's a lot of people saying goodbye to the ones that are going to leave the city. In the following shots, you can see Jack and his friends playing cards inside the pub. If you look through the window you won't see anyone. In the next shot, when Jack leaves the pub, the crowd is there again. (00:23:15 - 00:25:05)
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)
Continuity mistake: The design of the ship's bow changes significantly while at sea. The white form on top of the bow used for mooring first does, then doesn't have an indentation on top, possibly with a bolt in the center. The point where the curved railing meets the straight side railings either is or is not connected by a top horizontal bar, and the gap between the vertical posts changes size. Another mistake on this site mentions the anchor design change, but also the metal walkway is made of tubular metal bars -or- flat bars that are more numerous and closer together. (00:30:15 - 01:21:00)
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.