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.
This popped up again with ACR on the 5D2 - triggered by a bright LED sign.
Demonstrative pictures
Resizing tends to play down maze artifacts, so view at full size to enjoy them properly.