If you have a fair amount of server lag:
Put an item or stack of items from your hotbar into your inventory.
Quickly open a chest.
The items can duplicate under these conditions, with the stack in your hotbar reappearing.
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Can you provide a video of this happening on an unmodified 15w42a server, showing the items being duplicated and then dropping both the original and the duplicated set of items on the ground?
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Unfortunately I cannot. It's one of the most unreliable bugs I have seen, but it does exist.
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Sorry, but
I've had reports of it happening in survival
is not believable enough. If this does happen to you personally, please do create a new ticket with all relevant information.
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Fair enough.
It's not just creative mode; I've seen MC-5415 happen as well. This is across gamemodes. I've personally experienced it in creative mode, and I've had reports of it happening in survival. Furthermore, I had no trouble with this before 15w42a. Thus, this is a new bug and should be treated accordingly.