In this particular case, we’re talking about there being a difference between the Adobe Acrobat preview (with overprint set to be honored) and the printed result. The inappropriate use of overprint is an issue by itself. When spot colors are expected, the same color might appear twice or more, each time with a different name. Typically printers who ask for pure CMYK files get PDF files with spot colors in them.
The PDF file contains an incorrect number of spot colors (30+%). There are problems with transparency (30+%). It can also cause the spacing of characters to be completely wrong, with characters partially overlapping each other while others have too much spacing inbetween them. This can lead to text getting printed with a wrong typeface. Fonts are not embedded in the PDF (40+%). Unless this is corrected a thin white line may appear between the paper edge and close by images or tinted areas. Many printers request CMYK files so any PDF file containing RGB or Lab data will be incorrect, even though modern workflow systems and RIPs are perfectly capable of properly processing such color spaces. Use of incorrect or unwanted color spaces (50+%). When the resolution is really low images get a blocky or ‘pixelated’ look and straight lines that are not perfectly horizontal or vertical will show a staircasing effect. Low image resolution leads to a loss of sharpness. The resolution of images is too low (70+%). 
I’ve included how many people reported seeing each issue. Below are the main issues reported by the 1100 respondents. In 2017 GWG, a graphic arts association ran a survey regarding the use of PDF files in the graphic arts industry.
Even though the file format itself is reliable, PDF files can still be troublesome. The file format is also frequently used for submitting adverts to publishers or as a graphic file format for logos and drawings. PDF files are used to send print-ready data to printers.
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