If you shift-click a stack of less than 64 of any stack-able item into an anvil, when there is already at least one item of that type in the first slot, a stack with a red "0" will remain in your inventory. Clicking on it, or closing the inventory will make it disappear.

This also works with the second slot.

Steps to take:

• Get any number of stack-able items into inventory.
• Open the anvil menu
• Place any fraction of a stack into the first slot
• Shift-click the remaining items into the anvil
• You will see a stack of zero in your inventory

Sometimes you can also manage to see duplicated items (they are phantom, and will disappear if touched) but this isn't something I could reliably reproduce.