I'm proposing to blacklist w3schools.com on Stack Overflow. Answers linking to it are usually bad and often even link-only. Such answers also attract w3fools-related comments which would be fine with an explanation why the w3schools reference is bad in the given case, but lots of people just post a comment linking to w3fools without that comment.
Preferably that blacklist entry would not cause the default message to be shown but a more helpful one mentioning that the site is not an acceptable resource due to it teaching insecure programming habits (such as sql injection).
Somewhat related discussion: Discouraging w3schools as a resource