Cave spiders can hit the player through the floor.

As mentioned in the title. To test it, build something like in the screenshot attached. Spawn a cave spider on the ground a few blocks away. Now, unless you stand directly in the middle of that platform you'll get hit by the spider trying to climb to you. If you stand on any of the blocks on the edges or corners you'll get hit. You can even stand half in the middle and half in the edge/corner and the spider will still hit you.