From Mexico.purplecow.org

An example of 'maze artifacts'

Summary

The shooting conditions that brought this one involved heavy flash in view of the camera - obviously the 450D's sensor really didn't find it amusing. Sample has been contrasted up a little to show the curious nature of this one.

Using dcraw with the -f argument renders a less contrasty but artifact-free image.

Demonstrative pictures

Resizing tends to play down maze artifacts, so view at full size to enjoy them properly.