Always spell check your résumé and cover letter! Aside from demonstrating that you are careful and are able to make good written presentations, the presence of spelling errors may be a quick way for some people to cull your résumé, no matter the content.
While it may be printed later, your résumé is likely to be read initially on a computer screen - as such, make sure any formatting in your soft copy does not distract from the message you want to convey in print.
If you are modifying a saved document to use in applying for a role similar to another role you have applied for previously, make sure you change your application appropriately. A cover letter addressed to Mr Johnson of ABC Company probably won't be well regarded if you're in fact this time applying to Mrs Smith of XYZ Company. Check to ensure correct Mr / Mrs / Sir / Madam references as well as the correct spelling of the contact person's name - the last thing you want is for a person to stop reading your résumé at the second word!
Avoid scanning résumés, saving in unusual file types or changing page margins dramatically. These can end up as excessively large files, unprintable, or even unreadable on the employer's computer. Saving in .doc format is usually a good idea. If your résumé utilises graphics, the pdf file type is recommended to reduce the file size.