The give command will stack any item

When specifying the quantity of items produced by the /give command, these items will be stacked, regardless of item type.

This will result in strange and normally unachievable behaviour. For example, the command will allow you to stack items such as doors, beds, armour, tools and potions. These can create problems, as brewing stands normally only allow 1 bottle per slot, but will brew the entire stack at once. With an anvil, a stack of tools or armour can be repaired using the experience of a single item.

UPDATE:
This can also result in invalid redstone signal strengths (see MC-8257 for details).