I haven’t had this problem in a while, but it’s started to happen again.
For some reason, regardless of the type of light I use, it is showing through the ceiling in a certain render.
What causes this problem, and how can I assure that it doesn’t happen anymore?
If you are using a standard lightsource with Raytraced Shadows on, make sure that you turn raytracing on in your rendering panel. If you have a spotlight, and use Buffered shadows, make sure the Shadow toggle is also on in your rendering panel.
Never heard of the problem. Maybe if you supplied a picture of the set up or what the renders look like, then we could assist you.
Oh, and also make sure that you have your normals pointing the right direction. If you have a celing that only has one side, and the normal is pointing the wrong way, or if you have a two sided “thick” ceiling and both normals are pointing the wrong way, then you gotta fix that.