A good test is to run a simple test many times. Occasionally this finds things like memory leaks, file handle leaks, temporary file leaks, and so on.

If you have a simple web server for static content, and it works fine for a few requests, and seems fast when serving a thousand requests, you could try running a million requests. Or a billion. Not necessarily by many concurrent clients, although that'd be a good test too, just sequential requests.

I found a limitation in ab that way.