The rubber band effect in Mac OS X (10.7+) appears when you move the mouse up or down in a document and reach the end. The page will jump up and then back down. You can disable this feature with the following command:
defaults write -g NSScrollViewRubberbanding -int 0
Restart your Mac or logout/in to make the changes appear, or restart apps on an app-by-app basis to see the change.
Return to normal behavior with:
defaults write -g NSScrollViewRubberbanding -int 1