Not sure what causes this, but on a Mac Running 1.4.2 the shift key function is held down when you are not holding the shift key. The temporary solution is to restart the client completely. I have had the problem 3 times in the past couple of hours.
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Ezekiel
This bug happens to me too on Linux Mint. (I'm using the NEO 2 layout). I think this might be related to caps lock.
It's temporarily fixed when you press F11. I have updated LWJGL and it doesn't make a difference.
kumasasa
Consolidated this to MC-886. May not be the same issue, but stronly related
The solution is to press the right shift key. The cause is unknown.