I have narrowed this down to the fact that it is a force switching issue. You should be able to reproduce this easily on DayZ Experimental server SP 128.0.112.57
If you are holding a gun in your hand and you select a different gun from the quick select menu, you will see that it will swap the gun you are holding for the other gun. But now you will not be able to use that gun. You cant drop it, load it, holster it...
All you can do is quit the game and restart.
Must be related to the fact that you can not arm a tripwire either.