Have you ever gotten a call from your print rep saying there is something wrong with your output files? With deadline pressures and a budget to stick to, it can be a truly stomach-turning experience. Print publishing software such as InDesign has excellent pre-flight tools which help graphic designers to check/prep a document to print correctly. But sometimes there are problems pre-flight doesn't catch.
For instance: a rogue color is left over in a linked Illustrator file when the project started out as PMS colors then was switched to CMYK mid-project. Pre-flight will tell you how many channels of color that are in the document but it won't tell you where those colors are. That PMS color is probably underneath another layer and invisible. The graphic design document should only be four color, but you still see an extra PMS color listed in the colors used palette. That additional PMS color will mess with the print plate production.
If you can't see the extra PMS color in a regular printout then try this neat trick I discovered:
Print out the project in question directly from InDesign with separations turned on. You will get black and white print outs of each channel or "plate" of color so 4 pages for the CMYK colors and additional pages for each PMS color. From the print out you can pinpoint the location of the rogue PMS color. You can then go back into your graphic design documents and eliminate that color. Simple solution for a major headache.