In multiplayer, say your were crafting a redstone block with the drag and drop feature added in 1.5. When you click on the redstone block to receive it from the crafting bench, instead of getting the block, you get the raw materials put back into your hand as if you double clicked. same goes if you place the items individually. Unless you go at an extremely slow pace, the item won't craft. also, shift clicking to receive the item also gives you this problem. Various things are effected when doing normal inventory management as well.