Yesterday, I downloaded a referral link from a music site I'm working with. The elements were in upper case. The ampersand was uncoded. And the img element had neither an alt attribute nor a self-closing tag. Not surprisingly, putting the link on my web page caused it to fail XHTML validation.
After making the fixes in the code, the link worked perfectly and my web page tested as valid. I sent the site an e-mail with an explanation and the corrected code, but now I'm wondering if I should send them a bill, too! ;)