hi, i was playing around in a chest on SMP and realised with a series of rapid clicks that i could duplicate items. it took about 5 click in rapid sucession to do, but the item i had in my hand became 2. if i clicked too little, it would not happen and too many clicks would cause the item to dissapear, but i went on to find that i could "transmute" items into others i already had, using only 3 quick clicks.
i would (inside a chest) pick up the item i wanted more of, then hover the cursor over the item to be transmuted. i then clicked three times, very fast, and the amount of the item i wanted would be doubled, removing the "transmuted" item. this made for easy construction of diamonds if i already had one.
just to be sure, i then checked this glitch in single player, and found that it only works in multiplayer, and in chests. hope that this post helps to fix the bug!