Testing a new data-level validator for RDF and just found that Dan Brickley has a problem with his LiveJournal FOAF export, which states that he has the empty literal "" as a value for the InverseFunctionalProperty foaf:icqChatID.
So Dan, if you're reading this, I don't blame you (although I do perhaps blame LiveJournal for not having more rigorous data-checks... they do export millions of FOAF files after all).
Overall though, I blame the lack of a tool that you can use to check for such problems. If you get people hacking away on their RDF documents problems will arise in the data (I'm as much to blame as anyone else). You need a tool to assist you in debugging your RDF on the data-level.
We're working on it.