[Tilesathome] Coastline and ocean leakage in Iceland

[Ævar Arnfjörð Bjarmason]

A bunch of tiles covering Iceland are inappropriately either blue or
white squares on land or near the coast:

http://openstreetmap.org/?lat=65.47&lon=-22.71&zoom=8&layers=0B00FTF

This also happens inland:

http://openstreetmap.org/?lat=65.396&lon=-14.227&zoom=9&layers=0B00FTF

I can't see what the problem is, here's perhaps a better example where
an area of the ocean is rendered as if it's a landmass but there's no
island (or anything else but the ocean) in that area:

http://openstreetmap.org/?lat=65.7839&lon=-23.8626&zoom=13&layers=0B00FTF

What's the problem?