I can see something like this happening, but it would be a freak occurrence. I could see a case getting mislabeled or a case of Collection 1 being mistakenly put on a pallette with cases of Preview figures, but it would definitely be an anomoly.
I actually got about a dozen of the SAGA figures early in 2002. The store (Boscovs, an east coast department store chain) left them in a box that was wide open on the floor off the toy section. This was about a week to 10 days before the street date.
Figures have actually been leaking for some time. Hasbro employees get them early, and they've been trading with other toy industry insiders. But for figures to show up at retail? That is definitely strange.