A new discovery of tetrapod tracks, 18 million years before the first tetrapods were supposed to have taken their first steps (from the Middle Devonian, about 395 million years ago) is shaking up the paleontological community. These tracks take place earlier than any skeletal remains we have of creatures that could have made them; in fact the very earliest tetrapodomorphs couldn't even have made them, making the time depth all very confusing. These tracks were found, depending on which estimate, up to 50 million years earlier than they should have been. That's when skeletal fossils of creatures that could have moved using that locomotive pattern are first known. In addition, they're also completely in the wrong environment. It was believed that tetrapods developed in freshwater environments; deltas, rivers, swamps, etc. These tracks take place in what was a very shallow marine environment; tidal flats, essentially.
Skeptics wonder if they were actually made by some splinter group that converged with tetrapods, but then went extinct without giving rise to actual tetrapods.