PHP

No matter how many articles I read about XSS attacks (a.k.a. cross-site scripting attacks), I can never get enough of them. Here's a good one from InformIT that narrates a hacker-for-hire's attack on a client's web application; he starts with a simple login form and ends up pretty much in total control after a short amount of time...

When I've contributed to projects with fewer global variables, I've noticed that the flexibility and maintainability of the code has been significantly greater, and I have more fun working with it. My theory: if there are no globals, we can predict that an object doesn't play with any toys that we haven't given it.