If (as sugested by comment) a comment to scai's answer) this was a database problem, it could have been related to the rendering problem that occured after the node numbers reached the signed/unsigned 32 bit boundary. The nodes in the ways making up the boundary were created just after this occured (10 Feb 2013) so their tiles may have been affected in the same way that some of mine were. ( see http://www.openstreetmap.org/user/pieleric/diary/18588#comments )

If (as sugested by comment) this was a database problem, it could have been related to the rendering problem that occured after the node numbers reached the signed/unsigned 32 bit boundary. The nodes in the ways making up the boundary were created just after this occured (10 Feb 2013) so their tiles may have been affected in the same way that some of mine were. ( see http://www.openstreetmap.org/user/pieleric/diary/18588#comments )