Exec($_GET
As a theoretical aside, I wonder if it'd be possible to have a typesystem based solution to these kinds of problems - where variables coming from the user (or from another program) are considered 'unsafe' and the compiler refuses to let exec() or whatever use them until they've been through a cleaner/tester of some kind...  (OK, I know PHP doesn't have a compiler as such - but a static checker of some kind could work the same...)