When trying to convince the customers on the importance of Performance Testing I've found it's very useful to tell stories of disasters ocurred to other companies because of not running performance tests.
I invite you to share that kind of stories in this forum (just the title and web link), for giving our customers more reasons for investing in performance testing.
These are two good examples:
How the glitch stole Christmas?
Major retail Web sites experience intermittent outages, with peak period imminent.
Paris Hilton burger cavort crashes website
And a previous post in this forum has another good example:
I'll be waiting your cases.
Re: Performance Disasters
This one was nasty. The article doesn't say it was a performance problem, but I know that it was caused when a threshold of users (5000)? Attempted to log in. The system couldn't handle that many sessions, and crashed.