Their website says this:
You Returned a Personal DVD
Q: I accidentally returned a personal CD or DVD. How do I get it back?
A: If we can identify that a CD or DVD does not belong to us, and we recognize the customer who sent it, we will automatically return the disc.
If you return a personal DVD of a movie we carry in our own inventory, our systems unfortunately will not recognize the disc as belonging to you and we regrettably will not be able to return it.
Seeing as how netflix has EVERYTHING, odds are that I returned something in their inventory. Bummer. I think. I may never know, though. SUCK!