I work for an ad agency and it still happens to me once in a while, mostly with magazine and newspapers coordinators and even sometimes with supposedly skilled prepress operators. Goes from not viewing the PDF with "Overprint Preview" turned on to "your PDF has tiny white lines everywhere" (atomic regions). Even had one of these smart newspaper prepress guy rasterized my PDF in Photoshop (the ad got printed that way). All I supply is CMYK PDFX1a...
Better train people and risk they leave - than do nothing and risk they stay.