Injection attacks - spread it around!
It's very disturbing how few web application developers know anything about injection attacks. Like Bertrand LeRoy noticed, web forums are full of questions accompanied with code showing that the person who wrote it was completly clueless in that matter. So here comes Bertrand's article explaining those problems. It's very well written and should be easy to understand for everybody who knows anything about web applications and web programming.
Spread it around and make web a safer place!