some backpack slots behave erratically sometimes. at times it is impossible to transfer items from one slot to the inventory. when this happens the issue tend to replicate to some adjacent slots, eventually rendering the backpack unusable. restarting the game sometimes fixes it (sometimes that doesn't work and it's only possible to clear the bugged slot after some time)
also sometimes items would go to unpredictable item slots when shift+clicking (like the necklace, and ring equipment slot...and i mean regular ores like coal or lapis)

I'm using the latest version on MC 1.6.4

already fixed.

I'm using v. 4.2.2 build 19 and the issue remains

Also using v. 4.2.2 build 19
As you block the inventory slot in quickbar that holds the backpack to not be able to take it off when opened it looks like the problem is related to this, because:

The blocked slot in the backpack is always on the first line of the backpack and the same slot that holds the backpack in quickbar.

Hope my description is somehow understandable :-)