Deliberate mistake: When Jack and Rose are adrift in the ocean, the stars behind them are not the right ones for that time and date. In fact, it's not any sky ever seen on Earth. The right half of the sky is a mirror image of the left half. The fimmakers deliberately positioned the stars to form the shape of the pendant 'Heart of the Ocean.'
Continuity mistake: When the band goes out on deck to play as the ship is sinking all they have on is their uniforms. They are seen this way a few times, in the last shot of the band they all have on overcoats.
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.
Factual error: Professional Radio Operators hold the key used for Morse code between their thumb and two fingers - they don't tap it, as was shown. Tapping would produce a harsh voice in Morse code.
Continuity mistake: When the captain looks at the bridge sinking, the water level is by the middle of the helm wheel. In the next shot, from the other entrance, the level is lower than that, and in the next it's again by the middle of the wheel, which is also turned to a different position.
Revealing mistake: Throughout the whole movie, you can see the outline of hills in the background, even though the Titanic sank in the middle of the Atlantic.