pages slide right

Sometimes, I move my Magic Mouse and the Safari or Acrobat page I'm working on slides off the screen to the right. (My dock is on the right side.) The page is not in the dock and gone forever, it appears. This is not a "swipe between pages" because a swipe to the left does not bring it back. It does not happen often enough for me to notice my finger positions on the mouse so I don't know what they usually are. As luck would have it, though, this is the second time I typed this page because the prior effort exited stage right, laughing. It's gone. I was lightly holding the mouse with two fingers and a third finger was touching the top of the mouse. My questions, then: Where do the pages go when they leave? Are they recoverable? How do I not have it happen? Thanks (I use OS 10.8.4 and Safari 6.0.5. The mouse is about four years old.)