If you open your inventory, let your mouse hover over a full stack of items, and hold the item-drop key (Q by default), you will be able to drop all of the items one by one in quick succession. However, upon picking the items back up, many of them will have dissapeared. This only occurs in creative mode, and only when dropping items from the inventory UI (it doesn't seem to happen through the hotbar).
 
Steps to reproduce:
 
1. Enter a creative world
2. Obtain a full stack of any block or item that stacks to 64
3. Open your inventory and hover your mouse over the stack of blocks/items
4. Hold the item-drop key (Q by default) until you have dropped the whole stack of blocks/items.
5. Upon picking up the items you have dropped, check the amount of blocks/items you have dropped: only 14 of the original 64 will remain.
 
This bug also applies to blocks/items that are stacked to less than 64, and it doesn't have to be a full stack of items either. As I have previously stated, this only seems to occur in creative mode and only when droppiing items using the inventory GUI.