There seems to be an obscure problem when renaming objects. If you rename the object the same as another object which you deleted from the project it would cause problems. At one point my "light" just appeared as a white square rather than a centred gradient and wouldn't fix til I renamed the objects back to their original names.
Seperate to the above problem is that I changed a texture and it didn't update til the second time I previewed the game.
Sorry for being vague, I will try and upload an example for the first problem!