I have created a cube with several files. The cube then opens up to reveal objects inside. It was working great, then it suddenly changed to where I can see through the walls of the cube (the walls are still solid) and clearly see the objects inside. The walls I don't know if I hit a short cut key that enabled this, but I can't get it to go back. The problem still is there if I render.
A separate but related question regarding 3D and text. When I add a behavior to the text, it loses it's 3D appearance. That is the text is still behind something in 3D space, but it does not take on that appearance in the camera.
It sounds like the 3D group that you probably have your cube walls in is getting rasterized. This means that it's forced to comp into the scene in layer order (i.e. the order in the Layers List). It will still respect the camera and be depth-sorted unto itself, but it will get layer-ordered with the rest of the project. This is where knowing the order of operations comes in handy, as well as looking over the "rasterizing" section in the manual, which lists the various things that trigger rasterizing.
Did you happen to drop the group's opacity, change its blend mode or apply a filter to it? One way to get around the issue would be to put the cube's contents inside the same 3D group as the cube's walls, if they aren't already.