Mixing Inks (Process Blue vs Process Cyan)
When you mix, remember Process Blue and Process Cyan are not the same color. This problem is brought-up at almost every trade show.
Just look at the two colors in your PANTONE Formula Guide and you will quickly realize why your mixed colors were incorrect. This may be overly obvious, but the error happens more often than you may think.
By the way, remember that PANTONE Formula Guides should be replaced annually. The cost of a Formula Guide is far outweighed by just one job being rejected by an account.