Writing exams can be kind of fun. Grading them, much less so. But in any event, horror stories like this about old exams being recycled and found by some students would be enough to make me write a new exam every time I teach a course.
As for old exams, I put them up on the TWEN site for my course. That way, every student has access to them. Well, if they bothered registering for the TWEN site.