In 2004 three fossilized fish were found in the Canadian Arctic that were hailed to be an important missing link in the evolution of fish to tetrapods (vertebrates with four limbs).  The newly discovered specimens, called Tiktaalik, have been touted as one of the best proofs for Darwinian evolution (common descent), providing solid evidence for the transition from water to land animals. 

But now a paper in Nature is reporting that approximately a dozen fossilized tetrapod footprints have been discovered in Poland that date to 397 million years ago, at least 10 million years prior to Tiktaalik, forcing biologists to push the origin of tetrapods back about 20-30 millions years earlier than previously believed.  Tiktaalik cannot be a transitional form between fish and tetrapods if tetrapods existed prior to Tiktaalik!  Like so many other supposed transitional forms touted in scientific circles and the press (think Archaeopteryx and Ida), Tiktaalik is a dead end.