I've been contacted by them twice I think.
The first time was a rather intimidating phone call ("This is Unesco from Paris"), in which they stated (politely but firmly) that I had to clear the site of their logo and content from their website (their introduction texts). The second time was an e-mail, kind of a follow-up on the phone call to point out where I hadn't removed enough.
Both occurred some years ago. They are worried the most about the use of their logo I think.
They are also afraid of any errors, and that my site would look too much like an official site.
Copying images or info from an official site is never a good idea. See this disclaimer