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.
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.
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.
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.
Revealing mistake: This mistake, as far as I know, can only be seen in widescreen. After Rose, Jack, Tommy & 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.
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: The band decide to go their separate ways, just before the ship sinks. As they are seen departing, the lead violinist strikes up a solo - I think it'd be 'Nearer My God to Thee' or so the legend goes. A fraction of a second before he does, watch the second violinist - he disappears off to the right of the shot at first, then reappears near the side of the ship - he is obviously awaiting his cue as he turns round too early, i.e. before the lead strikes the first note, then turns away again.
Revealing mistake: In this scene, at the end of the film when Jack and Rose are hanging onto the railing, you can see the ship split in half, but when it drops, if you watch closely on the right side of the screen, you can see that the entire ship is still intact, the lifeboat cranes are visible all the way down the top of the ship.
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.
Revealing mistake: Towards the end of the film when Titanic is gradually getting higher Rose and Jack finally make it to the top of the ship, watch closely at the shot when they approach the top of the ship and Rose grabs onto Jack. In the background right at the very top of the shot you can see many green screens stood at the top of the ship in the background where people are running around.
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.