Shift-clicking the delete box in creative mode to clear your inventory while you have a goat horn in your inventory causes it to get "stuck" in the delete slot. You can still delete items by shift-clicking or placing them in the box, the the goat horn stays in the box. This also persists between worlds and after closing and reopening the game, and is still present as of writing.

What I expected to happen was...:
The goat horn would be deleted.

What actually happened was...:
The goat horn got "stuck" in the delete slot. While the slot could be used normally, the goat horn item was still there. This persisted between worlds and after closing and reopening the game.

Steps to Reproduce:
1. Use a goat horn so that it is on cooldown.
2. In creative mode, shift-click on the delete box to delete all items in your inventory.
3. Go to a different world or close and reopen the game to observe it staying.