Sometimes when I place down blocks I can see through all the way to the sky on one of the block's sides. If you'd like a screenshot I can get one for you.

Fixed in Ver. 0.6

Is this on adjacent Stairs in different orientations, or slabs that are next to each other? If so, this is probably a duplicate of either #15 or #16 which have screenshots.

Yes, it's like #16. Except I could see all the way through the ground to the skybox below if that makes sense