Does it involve any pistons whatsoever...?

@TheSpiderman2013 Spill the beans...
I've got no clue.

No wait! D: I'll keep asking for hints.

Does it have to do with entity collisions?

Hmm, I'm almost positive it involves entities in walls. Does it? (I've got to go for about two hours after this post, so I probably won't read your reply until then.)

@NutjobBob Pistons are required to make it, but will be removed when it's finished.
Hint: My trap was not possible before 1.9

Hmm ok since you could not do it before 1.9 that probably means that it involves the new blocks(or possibly mechanics)
You've denied the use of entities as they can be killed, but what about maybe shulkers(i know thats a longshot as it would take awhile to move them and all plus they are entities)
Maybe something with chorus fruit blocks and their hitbox size?
Oh or end rods and their hitbox size maybe!
I'll keep brainstorming!

@NutjobBob I did no such thing! (or at least I didn't mean to)
It does in fact use entities.

:D i miss read as you said

@NutjobBob Nope :)
(any entity the player collides with can be killed, making an inescapable trap kinda tricky)

Not that you werent using entities!
Ok now i have more room to brainstorm!

@NutjobBob Will you post a tutorial or something I'm so curious! Or at the very least tell me how hard it is to build/gather the materials?

Even a full inventory will eventually run out lol

@Triatomic How do you get the sand to remain an entity? Some 1.9 thing I don't know about?

Place down a 3 block high tower. The place a fence connecting to the side of the 1st block. Then place a piece of sand on the side on the side of the 3rd top block and it should fall on top of the fence as an entity.
I used to use it in some cool build designs, and i think that was how i did it.
Alternativly you could place 2sand blocks on top of the fence post then destroy the lower sand block.

@FishW Place down a 3 block high tower. The place a fence connecting to the side of the 1st block. Then place a piece of sand on the side on the side of the 3rd top block and it should fall on top of the fence as an entity.
I used to use it in some cool build designs, and i think that was how i did it.
Alternativly you could place 2sand blocks on top of the fence post then destroy the lower sand block.

Guess I'm still not up to date with the mechanics! Thanks for the overview.

@NutjobBob We're all to scared of you iwarriiori!

Here is cross-section of the trap

The sand is actually an entity hovering above the fences. This means that when you try to break a block you just hit the sand instead. You cant place any blocks because the entity is in the way. Ender pearls hit the sand sand entity and teleport you nowhere (I think). You cant jump out because of the slabs above.
It is actually possible possible to mine your way out if you stand on the very edge of a block, and face in one of the cardinal directions. However this is really difficult to do and will be impossible when taking damage ticks.

After much testing, I have come up with a recommendation for you and a conclusion.

First, you are right about ender pearls only teleporting you where you are, EXCEPT if they are aimed straight up and thrown. This lets them go through the slab roof. This can be fixed by adding another layer of blocks on the ceiling (corners too!).

As for the conclusion, this trap is inescapable assuming:

1. You remove space for chorus fruit teleportation and
2. The player cannot survive long enough to mine the wall and
3. The player cannot survive suffocation long enough to escape through the ceiling.

So just clog the whole place with obsidian and you should be good! (Actually not that hard tbh.)