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. (02:24:15)
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. (02:35:10)
The-ImmortalRevealing 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 exact moment when Cal steps out of the car just after the beginning of the movie, you can see behind him the back of Titanic still incomplete, with the internal structure totally visible. (00:22:06)
Revealing mistake: During the first meal on board, the make-up glue around Cal's hair-piece is very noticeable. (00:32:15)
Sacha
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)
Low CowRevealing 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. (02:30:00)
Movielover96Revealing 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)
Sacha

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)
Ssiscool
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)
leyesalot82789Revealing 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)
Sacha
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)
Sacha
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)
Sacha
Revealing mistake: When Jack and Rose are talking at the bow of the ship before the "I'm flying" part, if you look at the windows behind them you can tell it is a painted prop area and not a real ship.
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)
Ssiscool
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)
Ssiscool
Suggested correction: Maybe he didn't notice it?
Unlikely. Jack is seen trying to pull them apart. With the one link seriously weakened he would have had success earlier.
Ssiscool