Since the new update in which you are able to put water in stair/slab blocks, I noticed a glitch.

When 4 stairs are combined in a square shape and water is filled in the middle, it becomes immune to TNT. This method could be used on multiplayer servers to create easy TNT-proof bases, or in single player as well. This isn't an immediate issue, but I hope something is done about it in the future. 

EDIT: When testing this glitch more, I learned that it can just be a single stair with water inside of it. It works with slabs as well

Thank you,

kmsL