I noticed this when trying to craft a pickaxe or an axe in a crafting table. It only seems to do it with iron, gold, and diamonds. Wood and cobblestone work just fine. When dragging the iron, gold, or diamonds across the crafting grid while holding the right mouse button, the items will stay in the grid but when you let off of the right mouse button, the items will snap back to the mouse cursor and will not stay in the crafting table grid.