This article lists 5 mistakes.
For this list I can add two common mistakes advertises do quite often:
Buy advertisement on web site, get some users to click, but have incorrect URL on advertisement – users do not get forwarded to page you wanted them to go – money is spent on advertisement, no conversion to sales and potential customers get annoyed. All spent money lost and negative PR. This happens too often for various reasons (mistakes in ordering campaign, changes on target site during campaign, browser compatibility issues).
Have incorrect address on your product for more information. Or have some campaign on product to type code from product to potentially win something – and then the target page just does not work for customer – again frustration to user and negative view on company.
I have found out those mistakes far too common for many advertisers (big and small).