In the Original Series episode "The Conscience of the King", Kodos, known as Kodos the Executioner, has been the governor of the Tarsus IV colony. He ordered the execution of 4000 colonists, and only nine survivors (including Captain Kirk) were able to identify him. Kodos had gone into hiding as the actor Anton Karidian. Several of the surviving witnesses died under mysterious circumstances, presumably as part of a scheme to protect Kodos's identity.
My question is this: How would killing all the eyewitnesses protect Kodos's identity? Kodos had been a planetary governor, hardly an obscure figure. It stands to reason that there would be plenty of photographs available to just about anyone in the Federation. In fact, Kirk is able to retrieve a photo of Kodos (and one of Karidian) from the Enterprise computer. His appearance had changed in the intervening 20 years, but anyone who had seen photographs of Kodos and Karidian should at least suspect that they're the same person. (Facial recognition software should turn that suspicion into a near certainty, but that probably wouldn't have been anticipated when the episode was written in 1966.)
For a modern parallel, imagine Osama bin Laden trying to hide in plain sight as a touring actor. One wouldn't have to have been an eyewitness to recognize him.
My guess is that it's simply a plot hole, but is there an in-universe explanation?
One explanation that occurred to me is that (spoiler):
his daugher Lenore, who was the one killing the witnesses, was not entirely sane, and she therefore might merely have thought that doing so would protect her father.
But all the characters involved, particularly both Kirk and Spock, seemed to accept that killing the witnesses made sense.