Rivash 251 and Beis Yosef DH v’Nir’eh: Beis Din can punish according to the needs of the time, even without proper testimony. However, we are more apt to punish when there is greater testimony. We would cause a murderer to die (by locking him in a cell – Sanhedrin 81b) when there was not sufficient testimony against him. Some say that this is when the witnesses did not see each other, or there was no warning, or the witnesses disagreed about minor details. No one says that we do so based on one witness or women or Pesulim.
