When you have something print, it is always better to do business with a professional designer. They know their jobs ( normally) they have plenty of ideas of design that will look good for your company, they are good at making logos and everything.
The mistakes some make are hard to see and aren't directly in the design. So what is it?
If you guessed spelling mistake you guessed properly. Whether it is because they created a slogan for you or just copied the information you already sent them ( Name, address, etc) they may misspell something without even noticing it. This is why when they ask if you like the design always make sure that everything is written properly to avoid a spelling mistake that may cost you to re-do or re-buy what you bought because it looked unprofessional.