Minecraft Undetectable Anti-Player Trap

In all of my Minecraft life, I have never discovered a trap design as clever as this one. This trap is one of the only models that is actually undetectable without being extremely resource-consuming. This trap uses the concept that a player can pick up an item through the corner of two blocks. This item, when picked up, unclicks a wooden pressure plate, and the signal can be hooked up to pistons to drop the enemy into lava, TNT to blow the player to bits, or even just to a redstone lamp or a note block to alert you of the player's presence. If the item is picked up or if it despawns, a dispenser drops an item onto the pressure plate again and resets the entire trap. (Unless of course your trap was connected to TNT, in which case it would have blown up.)

I found a relatively short and explanatory YouTube video to demonstrate this concept. Click here for the video.