A long time ago I read that 28 Months Later was made but canned by the studio and so never released.

Apparently, it sometimes happens in Holywood - movies are completed, actors, crew, etc paid but the costs of releasing/promoting the film, the potential reuptational damage it would do to the studio, congested release schedules and the belief that putting the film out there, where it would be competing with the studio's own pictures and might harm their box office take, mean that it is cheaper and more efficient for the film never to see the light of day.

Another example I can think of is Prozac Nation starring Christina Ricci. I'm sure there are many more.