Steps to reproduce
On a flat floor, place a piston pointing up.
Power the piston
Two blocks away, place a piston on the floor pointing toward the vertical piston, such that when it extends its arm will touch the base of the vertical piston.
Spawn a creeper or llama between the pistons.
Power the horiztonal piston.
Expected result
The creeper or llama gets pushed against the vertical piston and glitches backwards into the piston arm of the horizontal piston.
Observed result
The creeper or llama clips into the vertical piston and down through the block below it.
When pushed against an extended piston, creepers and llamas can glitch through the floor.
Other mobs appear unaffected.
Related issues
is duplicated by
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I can confirm this on PE.
video: https://youtu.be/cLyE-qOpBsA
GoldenHelmet
This affects creepers and llamas because they are 1.8 and 1.87 blocks tall, respectively. The same thing happens when you push hoglins against a vertically extended piston head, because hoglins are 0.85 blocks tall.
[media]I conjecture that the reason this happens is as follows:
The piston arm is a full block wide at the end, and block collisions are factored from the top of the world down, so mobs that touch the end of the piston arm collide with it before they have a chance to collide with the piston base.
When the top of a horizontally-moving mob collides with a block, the game will try to move the mob downward by up to 1/8th of a block, similar to how players and mobs will automatically move upward when running or being pushed into half slabs at foot level.
The end of the piston arm is 0.25 blocks thick, so when it extends upward vertically its bottom edge is 1.75 blocks above the bottom of the piston. Most humanoind mobs are 1.9 blocks tall, which means their heads overlap the piston arm by > 0.125 blocks, so they don't get moved down when pushed into a piston where the base is at foot level and the arm is at head level. Creepers and llamas do get moved down because their heads overlap the end of the piston arm by < 0.125 blocks, so they do get moved down. Hoglins pushed into a vertically extended piston arm experience the same thing while baby humanoids do not, for the same reason.
Futher confirmation of this explanation is that you can get the same effect by pushing humanoid mobs that are 1.9 blocks tall into a trapdoor at head height. The bottom of the trapdoor is at 1.8 from their feet, and 1.9 - 1.8 < 0.125.
[media][media]
GoldenHelmet
The behavior described in my previous comment is also the cause of spiders and other mobs glitching through the walls of many popular trident killer designs (reported, e.g., at MCPE-158235 and MCPE-159339). If a spider is slightly elevated (by climbing or bouncing off of another mob) so that the top of its collision box extends just past the bottom of the second block up in the wall of the trident killer, then when a piston pushes it it will react first to the block at the top of its collision and be moved down and inside the first-level block. You can force this to happen by spawning a spider on top of a trapdoor in a 2x2 area and pushing it with a piston that extends next to the trapdoor.
[media]
You can also reproduce the behavior by suspending the spider at the right elevation with tp command and disabling the tp command when the piston extends.
Although this is easiest to reproduce with spiders, it probably also accounts for mobs like baby zombies and ravagers glitching through the walls of trident killers, or out the back of pistons.