My favorite one lately is when I have customers who know JUST enough about print to look at an G7 calibrated Epson proof and go, "this looks ok, but this photo here ((in a whole collage of them)) we'll have to tweak the colors on press." To which I have to politely attempt to get them to understand that no such thing is possible or is going to happen. it WILL be fixed before I send it to my pressmen.
the greatest one though was last week we printed some cards and it was supposed to have a yellow toned photo of some rubber boots. the epson looked "ok" but once printed the 7% cyan turned the photo a sickly greenish tint. the designer acknowledged that it was likely his file and that we were going to rerun them, but he told me that the colors would have to be adjusted on press. Long story short, after I edited the original photo, there was no Cyan in it anywhere before we reprinted. I love customers!!