Continuity mistake: At the very beginning of the film when Brock is talking with the big guy, they walk up stairs turn to the right, and right in front of them is a large piece of the ship with a no smoking sign on it, yet in the following shot, the large piece has turned into a very small piece, not even enough room for the sticker. (00:13:35)
Continuity mistake: When Jack and Rose have first met and are talking and walking on the promenade of the ship, in a wide shot of them, to the left of the screen stands a man with a hat in his hand talking to a lady. In the following shot his hat is on his head. (00:45:50)
Continuity mistake: When Will kills Tommy and commits suicide, the number of dollar bills on the floor next to Tommy's foot change number and position between shots. (02:19:40)
Visible crew/equipment: When handcuffed Jack tells Rose he's glad she found he wasn't a thief, a boom mike is reflected on the porthole's glass. In wider shots it disappears. (01:55:40)
Continuity mistake: When the Captain tells the morse operator to make an emergency call, the position of the papers and pencils in front of him move around between shots. (01:44:50)
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: 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)
Continuity mistake: 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: 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 scene after Jack and Rose try to save the child from the Italian guy, when the water bursts through the doors, they run away from it. But take a close look at Jack's neck: you can see the edge of a black wetsuit. And under his entire shirt, it's a little darker than it is a few moments later when they try to open the gate. (02:16:45)
Visible crew/equipment: When Jack and Rose enter the car to make love, cameras, shades, boom mic and studio structure are reflected on the brass parts of the car. (01:29:30)
Continuity mistake: About half an hour in when the rear of Titanic is shown, look at the very rear of the ship and you will see a red sign (near the triple screws) to the right of an over-hanging light. Shortly afterwards when Rose approaches the rear of the ship to jump off, the sign has now moved to the other side of the light. (00:31:50 - 00:36:15)
Continuity mistake: When Jack and Rose are being carried towards the locked gate by the water, we see the water and Rose hit the gate in the first long shot and then the camera changes to a close angle of the gate and Rose and the water have not reached the gate yet. (02:15:30)
Continuity mistake: When we see Rose at the start she says she doesn't know what all the fuss is about. As she says this she has her hand on her hat, but after the angle changes it isn't. (00:21:15)
Continuity mistake: Cal checks his pocket watch when getting ready to board the Titanic. In one shot he has his walking stick raised. In the next shot it has vanished completely. (00:21:50)
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.
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)
Continuity mistake: We see the plates sliding out of the holder and smashing as the ship starts to go vertical. Then 50 seconds later we see more plates falling from the same holder. However, the positions of the plates have changed and there are stacks of plates in sections that were empty. (02:30:10 - 02:31:00)
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.