LAST EDITED ON May-06-09 AT 05:19 AM (EDT)
 
Here's a rather tricky/complicated possibility.

In Twilight 7/"Fanfare for an Exiled Prince", Gryphon and R-Type quote a bit of dialogue from Ghostbusters, with the implication - at the time that scene was written, anyway - that Ghostbusters is fictional. However, we find out later, in one of the Raven stories (the title escapes me, but it's the one where she goes on a date with Etrigan) that the Ghostbusters do exist in the UFverse. ?? (Then again, there's a reference to the Spengler scale elsewhere in SotS, so this example may not meet the criteria after all...unless the UF-era Ghostbusters and the measurement borrowed from the 20th-century movie!)

-Joe-