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)
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migrated
galaxy_2alex
Cannot reproduce. Is the bug still persisting in the 1.7.3 pre-release?
migrated
Reproduced in 1.7.3!
-In creative, start with an empty inventory, and put a sword as the first item in your hotbar.
-Place a water bucket as the second item. Switch to your sword outside of the inventory.
-Try breaking blocks with the sword (you can't), but you can with the bucket.
-Back in your inventory, hover your mouse over the sword and press 2. Exit your inventory.
-Both will be able to break blocks, but one won't produce sound.
NOTES:The other way around doesn't work (hovering over item 2 and pressing "1"), the bug doesn't work in survival; however, if you do this bug in creative, then switch to survival, if you try picking up the sword in your inventory, it will turn into a bucket of water, and the other bucket of water stays as a bucket of water (you lose the sword). At one point during testing, I ran into another bug, where some blocks were "unloaded", or invisible, and didn't appear until after relog; the only way I could tell they were there was that water wasn't getting to them. Done on Windows 8 (if that makes a difference).
Cannot reproduce. Perhaps this has been fixed, as I am using the 1.7.3 release.