What a fantastic resource I just came across!
If you're a programmer, this is a highly technical overview of critical mistakes that can lead to disaster. They are the types of things I've had to be very aware of over the years when building Lottery Post, to ensure the security of each user's information, as well as the overall security of the site.
They are things that can turn a developer's hair white when they realize the back door's been open the whole time.
With many of the techniques that developers use, they are not even aware that they are building backdoors and major security openings into their software.
For example, many developers have no idea how to deal with passwords in their software -- both the paswords the software needs in order to access a database, or the passwords that users enter. This document points out those kinds of errors. It is extremely dense with information.