When a shulker box containing stacked unstackable items (such as armor) is broken, the items unstack and overwrite other items, and are partially also put into hidden slots. Once this box is placed another time and then broken again, the hidden slots disappear. A shulker box tooltip mod was used to display the items inside.

If the items are very NBT-rich, the player may permanently be locked out of joining the world/server due to the size being too large for the client ti handle. If it is dropped as an item, everyone within render distance is affected.

Image unknown-4: Shulker box containing various types of stacked unstackables.

Image 2020-12-25_20.56.30: The same shulker box after having been placed and picked up again.

Image 2020-12-25_20.57.46: Yet again the same shulker box, after having been placed and picked another time.