This is more of a historical slip-up than a theatrical one...

At the end of the movie when the frozen-survivors are in the water and Rose is waiting to be rescued, you can clearly see the reflection of the moon on the ocean in several different scenes. However, the moon was not actually up that night Titanic sank and would have risen after the sun that morning.

On April 16th, 1912 (the morning it sank), the moon phase was "new" and would not have been up during the night, at all. At the location where Titanic went down (41‹43ŒN, 49‹56ŒW-- off the south-east end of Newfoundland), the moon would not have risen until 5:50am and the sun would have risen before the moon itself.