It is important whenever you are working on a website to test it on various browsers and on various machines. For instance, the website may look fine in, say, IE, Firefox and Safari on a PC, but not good on Firefox on a Mac, as was the case of a website I was testing today.
(NB: I altered the content of the website in the picture... it's not really Production-Now.com's site)
Firefox on a PC
Firefox on a Mac
It is fairly common for websites to display differently on different web browsers--such as Firefox and Internet Explorer, especially because IE doesn't keep itself compliant with web standards--but to see such a vast difference in the same browser on different platforms was surprising.
I'm pretty sure the web guy over there has fixed the problem, but it was a good reminder to test your site on other machines and browsers.
Your Media Production Mentor