Traps for Vigilant Players?

Let's start with an extremely simple case: A dark corridor that suddenly opens up a hole in the ground.

Here is the redstone, all black concrete was replaced with sandstone:

All pistons are sticky, the repeater is set to the second setting.

As requested, here is a clearer image of the redstone, all unnecessary blocks removed and the tunnel made out of glass (which would of course be a really bad idea in the actual trap):

The hole in the ground opens quickly enough that you fall through when walking over it regularly and even mostly when sprinting and closes late enough that the trapped player does not get suffocation damage from the piston (in case you don't want to kill the trapped player).

Materials:

• 2 tripwire hooks
• at least 4 string for minimum visibility, 3 possible
• 1 redstone torch
• 1 repeater
• 10 redstone
• 3 sticky pistons

Advantages:

• no damage to player, if you don't want to
• It's pretty much a guaranteed kill, if you want to (drop into lava or other kill mechanism). Even enderpearling out does not work, because the hole gets closed over the player.
• items can be picked up or destroyed
• reusable

Disadvantages:

• This only really works in narrow tunnels. The idea can be applied for bigger holes as well, but that is much more complicated.
• sometimes fails when sprinting and mostly fails when jumping in a higher tunnel
• holes in the side of the tunnel can only be hidden with dark blocks, otherwise lighting gives it away
• resource packs can make the trap very easily visible

So, an simple but hard to notice trap is to chop down a tree, dig a 1x1 hole 2 blocks down, place 1 block of tnt at the bottom, (if you want to do more tnt dig out sideways where the tnt is supposed to go then fill that with tnt.) then put a pressure plate on the tnt and build a tree on top of it.

you can also try using scaffolding to create a very tall tower then making an bowl shaped structure at he top then filling it with sand then lava.

thirdly, you can make a plain with the same method described in the first prank, make the players want to mine the grass ( for example put a bunch of cats underneath the tnt to make a strange sound)

A very simple trap that is virtually undetectable is the "landmine" you have carpet covering redstone ore and an observer observing the redstone ore. Then you hook the observer to a bunch of tnt and when the player walks over the ore (covered with carpet) it will emit particles which counts as a block change so the observer activates the tnt and blows the player to smithereens. (just make sure when you are setting it up to wait until after the particles are gone before you add the tnt because the carpet being placed on the ore will cause the ore to emit particles) Heres a screenshot:

(Obviously you don't need that much tnt)

I have a special concept that also uses a hole to fall at: The Piston-Blocked Passageway

What's that, Mark Giraffe?

It is a redstone-activated hole that is eventually inescapable thanks to the piston blocking the way.

What makes it different from others?

It has to be set up in a 1x2 mining passageway (you can do it in a 2x2 one too, but not suggested) and what it basically does is that once you step on the unsuspecting pressure plate, it lights off TNT, making the block (I use gravel.) fall down and a piston will be activated alongside it, blocking the way, causing the player to actually fall off without fail.

We know that you can jump-sprint in the passageway to go faster which can lead to the basic falling block trap to fail, but this one stops that from happening.

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How do I make it?

Here's an example contraption in Java. I made the original one in Bedrock, but here, some minor adjustments are in place.

As you can see, a pressure plate is placed on gravel on a piece of TNT, which next to it is a piston (that is placed towards the direction of the player's destination). As it activates, it drops down the TNT and the sticky piston pushes the stone block to block the way of the player.

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^{This was quite a bunch to explain.}

Liquid Update Detector is an extremely sensitive mechanism that is very easy to trigger with changes that are undetectable by a lot of other means - and can be concealed in floor, walls and ceilings through use of any waterloggable blocks - including any stairs, slabs and trapdoors. Anything providing a liquid update to the block in question will trigger it - adjacent door, trapdoor or fencegate being open/closed, breaking or placing a block, placing a flower in a flower pot, entering a piece of string/tripwire, using a bed, opening a barrel, and many more.

This is a design by Jing RS Lin, but you can find a couple others or devise your own - the concept is fairly simple; an observer watching water in 'illegal state' (obtained by failing to provide liquid update by a retracting piston head that retracts while failing to pull a slime block (because its attached blocks are blocked by an unmovable block or over push limit). In Jing's design, the slime block would try to move the glass and the observers, but can't because the furnace is in the way, so the piston head retracts - and so the water fails to flow into the created space; now any update to the waterlogged slab will make the water to flow and trigger the trap.

As for the trap itself, a modification of the simplest enderpearl station is another easy to trigger device (just provide a block update) and a barrage of 16 splash potions of damage II is hard to survive for anyone. The dispensers can be hidden behind trapdoors which will open automatically when the device activates. Alternatively, you can load up TNT. In this case the dispensers can be pointed directly into solid blocks (concealing them and protecting them from the blast) and as long as there's empty space underneath, they will just drop - you can put a dispenser on top of a ceiling, pointing down, and it will dispense the TNT through the ceiling. Although - this is slower and easier to escape.

Note these traps are very sensitive - meaning easy to trigger while building. Make sure to "arm" (load with potions/TNT) only once you've verified your 'finishing procedure' doesn't trigger the trap.