I have this funny feeling that I've actually posted something like this before... but hey, it's an interesting topic.
As an ISV that works to serve the client's needs and there are many times where we are presented with a requirement where we KNOW it's a critically bad idea which would generally cause a lot of usability problems should we go ahead with the requirement.
But... the client doesn't listen, even though we were brought in there in the first place to advise them as well as develop the system.
Even worse is when the client just puts their foot down and says "You just do what we tell you to do!"
That's when we do what the client tells us to do, but we try our best to take the necessary precautions so that if the problems crop up it either doesn't hurt the system that badly or we can apply our fixes as quickly as possible.
That is... unless the problem is formed at the very core of the system and since everything is built around the core... there's no easy way to fix it once it goes down.
At times like this I really wish that I could just tell the client Look, we're trying our best to help you here, can you please just listen to us? We are looking out for your best interests too you know.