The bug

You are unable to crouch right next to world border, but when crouching and moving next to the border after it, speed of the player and texture of player visible from F5 changes like you are walking.

To reproduce

1. Go outside of world border 
2. Crouch and go one block next to the border

Alternate reproduction steps

/worldborder center 0.0 0.0
/worldborder set 16
/fill -16 100 0 15 100 0 bedrock replace
/tp -9.1 100 0.0

• Try pressing Shift and go near the border.

/worldborder center 0.1 0.0 

• Try pressing Shift and go near the border.

Expected result

The player would crouch next to the world border.

Observed result

The player does not crouch next to the world border.