If you time it just right you can open a Shulker box and stay in the GUI, land on it, bounce a bit and then fall through underneath. You stand on whatever block is underneath it or if you have no blocks underneath you fall all the way through. This only works if you land on the top of the box.
Linked issues
Comments 2
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However, this issue has been closed as a Duplicate of MCPE-28812
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Update: This bug does not work if your character is directly above the box (but does work if you jump and click when you're two blocks above.)