Client: "There's something wrong with your homepage template in Netscape. When I log in, I get an SSL certificate warning."
Me (after checking): "That's because you put your production server certificate on your test server. The domain is different, so this won't work."
Client: "No, the problem is in your HTML. Correct it immediately!"
Notably this client wasn't some totally web-dyslexic manager but the head of IT for a seven-figure project...