We already know that http://lmgtfy.com/ ("Let Me Google That For You" - a site that sarcastically demonstrates how to use Google) is banned from use in answers, mostly because a lot of people used it and its sarcasm, although funny, can be rude to some users.
Personally, I think that http://w3schools.com/ should also be banned from use in answers.
As demonstrated here, the site is riddled with errors and doesn't care about them despite having taken several measures to ensure that they always remain at the top of search results. To the discerning programmer, such errors are easily spotted and worked around, but to the beginner some of the coding practices recommended are not just bad but dangerous. Coupled with the similarity between "W3Schools" and "W3C", a lot of people will assume they are related - I certainly believed so for a significant amount of time!
I have seen answers to several questions where W3Schools is referenced, followed immediately by someone commenting with a link to w3fools. I think it would just be easier to prevent links to w3schools from being posted in answers to questions.