This hack is completely legal by Netflix's terms of service, so you're not risking mail-order-DVD-rental banishment by trying to weasel out a few extra DVDs.
In fact, it's Netflix's great customer service that makes this whole thing possible.
You see, when the next DVD on your queue is a less-popular flick that's not stocked in every warehouse, Netflix often has to ship your choice from a different shipping facility than usual—causing a delay in you getting your sweet paws on those discs. (You can sometimes tell that's the case when the first DVD on your queue says its availability is "Short wait.")
To make up for lost time, Netflix sends out the next available DVD lower on your queue so it arrives on schedule. You'll see it in your "DVDs at Home" list with a plus (+) sign next to it. In addition, your "Short wait" DVD is still on its way from the farther facility.
The result? An extra DVD in your hands. For free.
So if you want to squeeze a little extra juice out of your Netflix plan, try putting a few lesser-rented DVDs in your queue. Travel videos and older movies (from well before Netflix's beginnings in 1997) are a safe bet.