I think if there are circumstances that contributed to the mistake, Apple will take care of you. For instance, I accidentally downloaded an episode of Raines since it was the Free Download of the week. Only they failed to update the actual download page and I hadn't noticed that it wasn't free until after I hit the Buy button.
I contacted them through iTunes and they issued me a credit good for "1 Video". I still haven't used it. I'm actually tempted to try and buy a $10 movie to see if the "1 Video" credit can be used against it. Or maybe a $10 PBS TV Show since that's more in line with a "video" as opposed to a "movie".
In your case, maybe you could tell them that you bought the wrong movie (maybe you wanted the Italian Job or something), or that you hit the wrong button (you wanted to preview, not buy).
Can't hurt to ask.
ft