I was taking a look at Apple's "Mac OS X: An Introduction for Support Providers" (a very good overview of OS X, by the way), and I was trying to browse the file with the scroll wheel on my Intellimouse. It seemed I could scroll down, but then it would get stuck. I could scroll up again, but not down. Similarly, at the top of the page, I couldn't scroll up any longer, but I could scroll down.
It took me a couple tries to figure it out. In the Preview app, the scroll wheel only scrolls on the current page. To move to the next or previous pages, you have to hit the next/prev arrows at the bottom of the screen.
I'm not sure if this is a bug or a feature! I like the way you aren't suddenly jolted to a new screen if you scroll off the bottom, but it's a pain having to hit the arrow button for each page.

