I've noticed that depending on my zoom level or scroll position, additional rows appear on my maps. It seems to happen most often around 50% or 60% zoom.
In the attached images, I'm using the same map and the same zoom level. In the first show, you can see the two extra rows of white tiles at the bottom, but when I scroll down (or zoom out), those rows disappear. These rows also disappear if I zoom out further. It seems to happen pretty consistently when the edge of the map nears the edge of my browser window, but not 100%. If needed I can easily take a quick video so you can see it appear/disappear.
Happens in Firefox 18 and Chrome 24.0.1312.56 on Win7.
