False Constellations
This particular plot hole was discovered by renowned astrophysicist Neil DeGrasse Tyson. After having watched the film, he pointed out that the constellations that we see in the movie are not consistent with what the passengers of the Titanic would have witnessed in real life, considering their position.
He then provided Cameron with the correct version before releasing the 3D version of the movie, to rectify this mistake – although we doubt many people knew about it anyway.