If you are holding SHIFT and then you go to the inventory, you are no longer crouching, which is normal, but if you are on a block and are holding SHIFT and S to go backwards, and keep holding those keys and press E, you will no longer be crouching AND move backwards a little bit, causing you to fall of the block.
Linked issues
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Comments 6
Confirmed for 1.16.5. To fix: Make it so that if you open a UI while sneaking, walking should be cancelled first and then sneaking should be cancelled, then you wouldn’t fall off.
Can confirm.