It's in no way professional, but whenever a client calls up because they have an error on their site, it is so tempting to completely over-react over the phone:
"Oh my God! What did you do!? I have never, in all my years as a web developer, seen a site fuck up so royally! This is a disaster. We're going to have to wipe it and start from scratch! Hell, we might need a new god damn server!"
Sigh. A man can dream.
The best way to deal with them is to be excessively polite. Human nature makes it difficult to be rude at someone who is utterly polite.
It works out for everyone. They calm down, and it makes your day better too.