When in inventory screen, if you have let's say a stack in the mouse and do, over an empty item slot, a left-click and quickly follow up with a right click:

-In 1.4.7, the first action is done correctly (in this case, dropping the stack into the item slot), and the second action is also done immediately afterwards (in this case, quickly grabbing half the stack back). This is the correct behavior: a click done without any defined double-click behavior behaves as a single-click command.

-In 1.5 and 1.5.1, the first action is done (here, dropping the stack into the item slot), but then the second click is completely ignored.

This bug actually seems to be pervasive all across the game interface on all item slots, not merely inventory slots (crafting, smelting, etc.). It occurs both in Creative and Survival mode, and occurs 100% of the time (perfect reproductability).

Essentially, it seems as if the game tries to interpret the second click's short time interval to mean that the second click is actually part of a double-click action, and since there are no double-click actions involving the use of both mouse buttons, it just drops and ignores the second click altogether instead of using the single-click action for it.

Note that the second click is not ignored if you do the opposite: a right-click quickly followed by a left-click, will still do both actions. Same thing for two right-clicks in quick succession, or two left-clicks.

However, in the case of a left-click done quickly after a right-click, there seem to be a slight delay involved in the response time for the second click, that didn't exist before in 1.4.7.