Once redstone and logic gates were implemented, one of the first traps made was a pressure plate on top of TNT.
This made people discover two things: Pressure plates and TNT do not collide with anything when active.
Because of this, TNT traps could also be used for surprise drops, but at the cost of the landscape, which brings me to my idea: Decoy blocks.
What are decoy blocks? They are special blocks that take the properties of the nearest block*, or whatever more is near it(See below for a better explanation), but since they are made of iron, need to be mined with a pickaxe.
*doesn't include water. If there are no blocks near it, it'll take the form of an iron block.
If the furnace is the decoy, it'll look like the diamond blocks.
  
Saying "**** you" to friends, enemies, mobs
First usage listed
Easy to spam
And then there's the problem of detecting decoy blocks. You're positive some ******* filled your base with them, but you can't be sure. How do you distinguish a decoy block from a normal block? You pay attention. Decoy blocks should be sort of shiny and a bit lighter in color, so if you pay attention, you'll see it. And then Bob was paying too much attention to looking for decoys, that he didn't notice a pressure plate, stepped on it, and fell to his death.