The bug
The larger part of a shelf mushroom block placed at eye level is 1.5 blocks above the ground, the same as an upper slab. Therefore, it’s expected that you can sneak under it. However, the collision box and hitbox of the block goes as low as the smaller part, preventing this.
How to reproduce
Find a wall
Place a shelf mushroom on it
Try to sneak under it
You can’t
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