If you are a member of the application security community, you are bound to know this hilarious xkcd cartoon. It is so good that it found its way to non-expert circles. I once got it physically framed as a birthday present from friends.
Like most of you, I though that this is a great way to explain SQL injection. For most of us, this is what it is. For a few, it is a real life problem
My dear friend Or Katz published an even more hilarious blog post outlining the challenges of someone who happens to have a first name which is an SQL keyword. His post is also a very good discussion of the use, or rather abuse, of signatures for web application security. A great and worthwhile read.