Assume nothing. Test everything!
Just because you are paying for a 100MB connection does not mean that you will get that much. And if you don’t test it, you will not know until it is too late.
Just because your admins insist that your test environment is an “exact copy” of your production site does not mean that they are. And if you don’t test it, you will not know until it is too late.
Just because your web server was the bottleneck in the last test and you added another front-end server and put a load balancer in front of them, does not mean that you have doubled the capacity of your site. And if you don’t test it, you will not know until it is too late.
See a pattern here?
Chris
p.s. Yes, we have seen real customers fall into each of these traps. Fortunately, we were there to help them out!