Inventory/stacking bug

[HaZZoR]

Title: Inventory/stacking bug

Version: 3.1.2

OS: Win 7

Java Version: 7

Description of Problem:

when moving things around my inventory or more frequently stacking them, some wierd things happen, i find it hard to describe so i made a short video

http://www.youtube.com/watch?v=y9_7HhUGBHw&feature=youtu.be

Error Messages:

N/A

Error Log:

N/A

[HaZZoR]

just to add, i hopped into SSP and the problem doesn't happen there, so it's just SMP