Look at the attachements for example. I've got two iron ingots and two flints in my crafting menu. Now using shift + left click crafts two flint and steel. But there is only 1 CraftItemEvent called having the following content:

[11:59:20] [Server thread/INFO]: event.getCurrentItem().getType() = FLINT_AND_STEEL
[11:59:20] [Server thread/INFO]: event.getCurrentItem().getAmount() = 1
[11:59:20] [Server thread/INFO]: event.getCurrentItem().getDurability() = 0

I've also tested it with torches producing the same bug.

That is a problem if you want to modify every item beeing crafted, like reducing durability of flint and steel.