You know when you're playing SMP and it lags?
Then you find that you can't pick up items and put then in your inventory or chest,
but can only shift click them (without holding shift)!?

Heres the answer:
Hit the Left-Shift key 1x, see if that works if it doesn't then,
Hit the Right-Shift key 1x, and see if that works and that will usually work.
(tested and worked)

If none of the above work then log out and log back in, or restart minecraft.