While in your inventory, with items in your hotbar, quickly hitting the numbers while in creative mode can cause some items to overwrite others, and create a duplicate of one item, while removing the other. This does not work in survival.
Spam-clicking items causes same effect. Can sometimes disappear even in survival, see MC-4108.
Easy way to reproduce:
Open the inventory
Put two different items in slots 1 and 2
Hold the mouse over slot 2
Press and hold "1"
The items will swap a while and eventually the item of slot 1 overwrites the item of slot 2
Related issues
is duplicated by
MC-682
When clicked an item in hot bar a lot, it dissapears
MC-970
Stack Duplication
MC-1770
Armor Duplication
MC-2511
Armor Duplication Bug on MC 1.4.2
MC-4108
Items disappearing when moved in inventory
MC-4109
Items dublicates if you click on it several times
MC-4321
Creative Mode Survival Inventory Duplication Glitch
MC-5957
Duping Glitch/Bug
MC-7202
Inventory Item Duplication
MC-7693
Duplication glitch in creative mode
MC-8508
Bug when "number clicking" an item in the creative inventory
MC-11334
Disapearing/duplication bug
MC-11531
Spam clicking and the item amount changing
MC-15871
Graphical Glitch - Enchantment Table
MC-16604
Armor Double Glitch
MC-45036
Skulls Duping Upon Repeated Wear
MC-52640
Duplication Glitch-Enchanting table
MC-54410
Testfor named item changes item
MC-59158
Disappearance of blocks when it move with the quick buttons (1, 2, 3...) in the quick access bar (creative)
MC-59712
Rapid Clicking Item Duplication or Deletion
MC-61681
Disappearing blocks from hotbar in Creative.
MC-65812
New Enchantment Table Duplication Glitch [Creative only]
MC-66581
Moving items with the kyeboard duplicates in Creative
MC-67847
Creative Drop Duplicating Bug
MC-70309
Switching items with number keys in creative mode and trying to place the block makes it dissapear.
MC-71706
Creative inventory bug
MC-71939
In creative, items disappear when clicked then exiting inventory
MC-75452
Stackable items/blocks are duplicable in Creative Mode using the anvil
MC-75965
Quick block moving in your inventory in creative mode.
MC-79782
Items drop as different items in creative mode
MC-86035
inventory/hotbar shortcut duplicates item
MC-88577
Item duplication with switching hands
MC-89879
Moving a block using number keys, while in creative, causes the block to not be placeable.
MC-97946
Repeater Commandblock doesn't remove single items
MC-101999
Duplicate Boots (Chain, Iron, Gold, Diamond)
MC-106227
Shooting arrows with sword bug? (1.10.2)
Comments
You have to have your inventory open
http://www.youtube.com/watch?v=EDJ7fNwK3L0 demonstration of the glitch
kumasasa
Confirmed for 12w49a.
It's not necessary to jam on one of the number keys, it's enough to keep a key pressed for some time.
Sorry, can confirm in 13w42b
Confirmed for 1.7.2
Ezekiel
Is this still a concern in the latest Minecraft version 14w02c? If so, please update the affected versions in order to best aid Mojang ensuring bugs are still valid in the latest releases/pre-releases.
Confirmed for 14w18b.
Jeuv
Confirmed for 14w28a.
Confirmed for 14w28b
Confimred for 12w32a, b, c, and d
Confirmed for 1.8.
Sonicwave
Confirmed for 1.8.1-pre2. (I switched a minecart with a creeper spawn egg and upon right clicking on the minecart a creeper spawned and the inventory slot immediately changed to the creeper spawn egg.)
Sonicwave
Confirmed for 1.8.1 and 1.8.2-pre1.
Confirmed for 15w44b. Here is a slightly different way of reproducing this.
1: Put a stone and a sapling in first and second hotbar slot respectively.
2: While hovering over the second item, hold the 1 for a few seconds.
3: It will seem like nothing has happened. Hower, if you exit to the title screen and reopen the world, the sapling will have turned into stone.
A few points:
It shouldn't take more than 3 tries to reproduce the bug. If it does, put the two items in the opposite order.
It seems like the item with the lowest numerical ID always overwrites the item with the highest ID.
This bug works (seemingly) for any item, if the two items only differ in data value, the one with the highest data value will change to match the one with the lowest data value.
Confirmed for 1.8.9 and 15w51b.
Confirmed for 16w02a.
Confirmed for 1.9-pre1
Confirmed for 1.9.0
May be fixed on 1.9.1-pre3. Happened within 2 seconds in 1.9; could not reproduce within ~60 secs in 1.9.1-pre3.
EDIT: I just reloaded the world in which I did my testing, and the blocks are duplicated!
Confirmed for 1.9.3-pre3.
Only appears client-side upon reloading the world.
Confirmed for 1.9.4.
Confirmed for 16w20a.
I can't get this to happen in 16w21a (I did reload the world). More testing is required.
Confirmed for 16w21b. This happened without reloading the world.
Confirmed for 1.10-pre1. Doesn't seem to happen in the "Survival Inventory" tab.
Confirmed for 1.10-pre2.
Confirmed for 1.10.
FaRo1
I just reproduced it pretty quickly in 1.10, but then I wasn't able to reproduce it for a long time with different setups mentioned here. Does anyone have tips?
Confirmed for 1.10.1.
@unknown: Do not be in the Survival Inventory tab.
Confirmed for 1.10.2.
FaRo1
Again: I reproduced it by letting another program type 1.000 "2"s, but then I tried multiple times again and even changed the number to 10.000, but didn't get it. @unknown, how do you reproduce it usually?
I put four items with different IDs (not sure if necessary) in slots 1, 2, 3, and 4; then I spam the 1, 2, 3, and 4 buttons while moving my mouse left and right over the slots.
@unknown: Here's a video demonstrating the bug. Sometimes it is necessary to reload the world to see the duplication.
Unable to reproduce anymore in 16w42a, possibly fixed.
Tried all the aformentioned methods, as well as the method I used to use myself in the past, but it doesn't seem to occur anymore.
Resolved as fixed, please update if you can still reproduce this bug by providing proper steps.
Misode
Managed to reproduce this in 18w43c in creative mode. Please reopen.
 EDIT: I can reproduce this every time by just holding down a number key.
FaRo1
This has been closed for over a year. Can you please make a video in which you reproduce it?
I'll also try to reproduce it in ~10 hours, I had a pretty reliable way to reproduce it on my old PC, I could transfer it to the new one.
FaRo1
Confirmed in 18w43c, too.
Cannot reproduce 😞