Free World of Warcraft Exploit: How to buy multiple limited items

There are two ways to purchase an item in the vendor screen: Right-click on it, or left-click and drag it in your backpack. If you are fast enough, you can do both with an item that is in limited supply (only 1 can be bought):

Right-click the item you would like to buy and immediately drag a copy to your backpack. If you're fast enough you end up with two items. If you're too slow, you get a "The item was not found" message.