In 1959 Kravetz published a proof that the Teichmuller metric on Teichmuller space is negatively curved in the sense of Buseman. This was widely quoted and used until Linch found a gap in 1971.
In 1974, Howard Masur showed that the Teichmulller metric is not negatively curved, by exhibiting two distinct geodesic rays which have a common starting point but stay a bounded distance apart. There is now a whole subfield studying Teichmuller geometry, which grew out of the failure of Kravetz's theorem.
