By using the number keys you can make the game treat an item as another item, it has visually switched but still acts like the original item (in creative mode) or duplicates the item (survival mode). I originally found this with a water-bucket and sword.

To reproduce:

1) put a diamond sword in slot 1 of your hotbar and a water bucket in slot 2 of your hotbar and hold the sword in your hand.

2) press "E", hover over the bucket and press the "1" key to make the bucket and sword switch places.

3) exit your inventory and attack a mob. The bucket will deal a damage amount equal to a diamond sword to the mob. If you are in creative the items remains a bucket, if you are in survival the bucket becomes a duplicate sword and takes durability wear.

This confusion by the game persists, even if you use the sword (which still behaves like a sword). Attempting to place the water in the bucket causes water to appear for a moment, then the bucket refills and the water vanishes (if you are in creative and the bucket still looks like it exists).

If you do this the other way around so that the sword is treated like a bucket, the sword can break blocks in creative (but makes no sound)