1. Thinking that their editor is the same as a final proof and copy edit. Reality with books which are often very long is that an editor can't necessarily catch everything. Notice on most mastheads of magazines that you have several layers of editing -- the editor-in-chief, editor, managing editor and copy editor. The reason for the layers is that each role is slightly different. Even if you have a reasonably talented editor to clean up and help your copy shine, a final proof will still be a necessity. I try to tell writers this and often hear they don't need that final proof, which is completely wrong. When I end up showing them via a quick proof of a few pages how that belief is incorrect, it often opens their eyes to the true difficulty of getting a completely clean draft.
2. Not understanding that a Word document and a book template are two different sizes and produce a different page counts. I ask writers the size of the book, and they quote from the Word document. Standard book sizes (we use 6 x 9) contain approximately 250-300 words per page vs. a Word doc which contains roughly 500 words per page. You have to do the math on the Word doc's total to get the actual page count for the book template. The page count affects print cost.
3. Not understanding the value of professional services. When you hire an editor or graphic designer to work on your book you are hiring experienced professionals. If your focus is on getting something done cheaply then you have to remember my favorite saying, "Cheap is as cheap does." So when you end up with a document riddled in errors that you paid the lowest price to get done then you shouldn't complain.