From the as-of-yet-unpublished PBB Lexicon:
riff-punked (rif'-punkt) v. : To be fooled, after the first few bars of a song, into believing that Song "A" is in fact Song "B." This may be accompanied by singing the beginning of the lyrics to the wrongly-guessed song before shamefully realizing the mistake.
[Example: "Man, I totally thought the song on the radio was "Stan" by Eminem, but it turned out to be stupid Dido's original song. I was totally riff-punked."]