How do you figure that? D'Hoffryn mentions it in "Selfless."

I think the events in Villains are supposed to happen in chronological order. If we're shown on 29-th min. the pictures of the events that will happen onscreen in 10 minutes it means these pics appeared before the actual events happened.

Or maybe it was just a continuity error *shrugs*

Reference to the children's song "Puff, the Magic Dragon."

Thanks for the link - very educative! Still, I can imagine that upon hearing Spike's words, the dragon decided that Puff is his real name. He may be new to human civilization; he may try to adapt to human standards by accepting the first name he's called. :)