Bad SEO advice
As with all information on the Internet, there is a lot of good advice out there but there is also a lot of bad. SEO advice is no exception. I have read too many times that websites can be designed to be fully optimized for the search engines. The problem with this statement is that things are constantly changing so it is impossible to perform a one-time optimization. One slight algorithm change and your effective SEO is suddenly useless. So you must be very wary of who you accept SEO advice from and you must constantly stay up-to-date if you are doing any of it yourself.