From the things I heard in yeshiva category:
Are we allowed to place less revered seforim on top of more revered seforim? I believe that the consensus is that you are only allowed to stacks seforim in chronological order from top to bottom (e.g., Mishnayos above Gemara, Chumash on top of a Navi). Is this correct? Is this a halacha from Shulchan Aruch or other source, is this a minhag, or is this a vast conspiracy perpetrated by rebbeim?