There's a bug with using the Shift Button whilst in inventory. As I'm sure you're aware, by using the shift key in the inventory it will move the item to the bottom or top bar.
However, sometimes it seems to get stuck, this has happened on a variety of PC's/Laptops running either Windows 7 or 8. I'll be going about my business and suddenly I'll press an item to go and craft it for example, and it'll move to the top/bottom bar as if I was in shift mode.
This error has been around for many versions, it happened on my old PC too and it's not the keyboard getting stuck as I've used several keyboards in the time of this happening. Annoyingly, the only way to fix it is to completely close the Minecraft Client and re-open it or wait about an hour and it fixes.
It's very annoying, especially when trying to craft in a survival server and having to disconnect all the time.
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Duplicate of MC-886, please use the search function to see if your bug has already been submitted. Currently over 51% of tickets are being closed as duplicate.