When I was looking for R5 at my local GameStop they pointed me to two different stores that had them in stock. And when I got to one of those stores? The R5 I got wasn't on the pegs, and the clerk asked if I had a pre-order. I didn't have one, but he gave me an R5 anyway. I really hope I didn't take one that had been reserved for another customer, but GameStop should have some policies in place to make sure those pre-order reservations are fulfilled first.
When I reserved my R5 at the store they sent me a very non-specific e-mail saying "they would hold the item for a period of time". At first I assumed I had 24 hours to get my item, but when I re-read the e-mail it was so non-specific it made me think they could easily put it back out after only a few hours, so I hustled to the store and grabbed it ASAP.
Its the first time I've ever purchased anything from GameStop, or reserved anything from GameStop. That and the challenges I had with cancelling my pre-order lead me to believe its really not a terribly well run place without a lot of controls in place.
So I guess I might sort of feel bad that they gave you someone else's "reserved" item, but it could have been reserved for a week or it could have been reserved for a few minutes, and based on their policies it appears to be first come first served.