A pressman in Tulsa told me my file was fine, then he flipped the sheet over
I sent a job to a small press in Tulsa last month and the guy called me back saying the file was clean. Then he said he flipped the sheet over and the back side was printing my rich black as a muddy dark brown. Turned out I had two different blacks in the file, one from Illustrator and one from a JPEG a client dropped in, and I never noticed because my screen made them look the same. He said he sees it about twice a week and just laughed it off. How do you all catch mixed blacks before sending files out?
Ugh, the classic "looks fine on my screen" trap. Your monitor is basically lying to you because RGB black (like 0,0,0) and rich black (like 60,40,40,100) can look identical on screen but print completely different. I've started using the Output Preview in Acrobat with the "Show all" separations mode, that way any stray JPEG black or weird Illustrator black shows up as a separate patch of color you can actually see. The other trick is flattening everything and checking total ink coverage before export. Two different blacks in one file is such a sneaky problem, your Tulsa guy probably has a whole comedy routine about it by now.