This has been on ongoing problem but it has happened maybe six times in the last couple of hours.
I just take off walking one direction or another. I walk to the end of the street and keep on walking in place. At first I thought it was catching up with my overuse of the arrow keys, but a few times I have gone to get a cup of coffee or feed the dog and come back and I am still walking off the edge of the page.
I have to leave the world for it to stop - and that even takes several seconds on most occasions.
Safari 5.1
Windows XP