Is this problem intermittent and how does is manifest? I ask because I've just been working on a scene using a mask to prevent small fake stones appear on top of larger ones and had no problems at all even though I hadn't specified through which camera the mask should be projected. As the scene progressed and increased in complexity (more pops etc) I suddenly couldn't move the camera at all without a complete crash. A step by step removal of all nodes/pops/objs etc traced the problem to not specifying which camera to use for the mask. I can now induce or remove the problem in the original scene with all nodes in place. Is it dependent on scene complexity/memory use? and if you have to specify the camera why doesn't it default to the render camera?