This only happens in firefox with a touch gesture device like the magic mouse or a macbook/macbook pro, but on some websites (some I've done and some I haven't) you can scroll horizontally even though there is no horizontal scrollbar, and there shouldn't be a scroll bar, so it's scrolling to stuff not supposed to be on the page.
http://starryeyedmusic.com/events - scroll to the right (that sidebar is positioned off the viewable page so as to not be seen)
I've also had this happen on some other websites, they work fine with normal mice or on chrome or opera with a trackpad or magic mouse.
-Danny