You wouldn't believe how many clients, designers and agencies neglect to request a proof from the printer. This is a major gaffe on individual projects, and absolutely horrendous when it concerns an ad placed in a newspaper or periodical.
Offset printing is not an exact science; printers adjust density of cyan, magenta, yellow and black amongst the data on plate to arrive at a pleasing balance. That means if Verizon asked for a proof, and their ad is on the facing page from yours - your colors will sacrifice their intended levels to enchance the accuracy of Verizon Red.
There's more to it than that, but we'll leave details for later - or until you contact us for more info.