There's a leakage of the real world into the Dresdenverse, which results in Harry's entire life being suddenly narrated by James Marsters. All of the other characters can hear this, which ends up revealing some rather uncomfortable secrets...

Bonus points for Harry or one of his friends recognizing the voice -- they do have Buffy in the DF, after all, even though he sounds really different without Spike's British accent -- and trying to track James down.