Barcode streaks in some Mapnik tiles

I rendered the entire world from zoom 0 to zoom 10. The higher I went the more I noticed that various tiles had a series of streaks that look like barcodes. Some were black streaks on land and water. Other streaks were pink or green on land. In the latter tiles, it looked like the tiles were squished to half their size (squished to the left).

Any ideas why this might be?

The box used to render the tiles was a RHEL5 box and I used the Mapnik wiki.

– Kevin

Never seen that, perhaps talk with the mapnik people?

The same with me. The trouble maybe arises from RHEL5 (using Centos5)?
Have you allready talked to the Mapnik-people?

Also, with me it is not the higher I went, but the more I allready rendered (I was rendering from Zoomlevel 0 - 7) then restarting at 0 which got me the streaks already on Zoomlevel 0 whereas i did not have them the first time.