Bug report 688068 actually references two separate problems. This bug report is being created to track the second issue. The test file (in 688068 labeled 'A simple example showing stripping issue') shows a problem with overprinting. This file shows a rectangle with which is a combination of solid black and 50% cyan. The rectangle should appear as solid black. Instead it shows stripes of solid black alternating with haltoned cyan on a white background.
Fix described in: http://ghostscript.com/pipermail/gs-cvs/2005-May/005522.html