On any page longer than two screens, Page Up/Down move by two screens instead of one. (Most of the time. Sometimes, they work normally; there is no apparent pattern to when, and it has nothing to do with the page - it works correctly until I reload the page, then goes back to the buggy state.) Space and Option-Up/Down work normally.
Using build 41760 (but it's been doing this for every build for, I think, a couple of weeks) on OS X 10.5.6.
*** This bug has been marked as a duplicate of 24477 ***