Here's an interesting list of top ten errors printers encounter when they receive files from designers:
Top ten problems in file prep for print
I find that many young designers commit these errors more frequently than more experienced designers. I admit from time-to-time an error like forgetting the fonts can happen with experienced designers too. But by in large experienced designers are used to getting every aspect prepared for print, since they are so familiar with print (and printers). Whereas the younger designer, fresh out of school, is used to making graphics for web and interactive media. Therefore, issues like CMYK color prep, fonts, or bleeds are difficult to consider.