You have working titles and fake titles. For instance, Cloverfield used many, many working and fake titles. The real working title of Cloverfield was 'Untitled J.J. Abrams Project'. The first time that something of this movie was released (I thought it was a teaser trailer) was carried by the name '1-18-09' (the release date of the movie). But during production, they where using all kinds of fake titles so that people who saw them shooting a scene, couldn't find anything about it on the net. So it's also a way to keep a movie more of a secret.
Used 'fake' names for Cloverfield: Slusho, Clover and Cheese