If you have a look at the attached screen shot you will see a valid tile (highlighted with firebug) and several broken (pink tiles). The broken tiles are offset in the X and Y direction and are broken because they are referencing an invalid path in the kamap generated cache.
Some other observations:
-All the broken tiles are always in the same row OR the same column
-The broken tiles always load after all the tiles in the viewport have loaded successfully. (ie. an extra row/column is loaded with broken tiles)
-So far I have been able to reproduce this bug in Firefox 2.0, IE 6, IE 7
In the meantime to not display broken pink tiles, I have added a fix to tile.php which returns a single pixel image if the specified image path deosn't exist.