Testing scripts on a development server is one thing - a controlled environment, a set number of "users" who know everything about the application, and not much load on the actual machine. The result is too often scripts and applications that just aren't written to handle the stress of real life use. One key element of this is the application server the app is on, and LinuxJournal has a nice piece on some stress testing for it.

They focus in on the Apache web server, giving links to the tools that you'd need to hammer away at your Apache server. As you have probably guessed, finding where our optimal performance lies and pushing those limits is more than a minor challenge. Like the formula-one race car that runs with almost absolute technical efficiency, the five main components of the Web-based application determine whether the system can handle its load optimally.

It can never hurt to stress-test your applications and your servers, especially before a major update or a server change. I've seen some sites introduce a new feature and it tank the entire machine just because it can't handle the load...