Steps to reproduce

1. On a flat floor, place a piston pointing up.
2. Power the piston
3. 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.
4. Spawn a creeper or llama between the pistons.
5. 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.

Demonstration video