RETCON 1: K A R M A
Retroactive Continuity or "Retcon" is the practise in works of fiction to subsequently alter a previously given "fact" of the fictional world, and offer an explanation as to why the earlier apparent "fact" was wrong, misunderstood or subject to a new interpretation. RETCON is a dangerous practise in storytelling because it can breach faith the reader or viewer. In gaming it can ruin an entire game beyond repair. HOWEVER, sometimes it can be used, sparingly. Retconning began to be used in comics when the continuity of a storytelling world became too complex to be easily maintained. It became easier to handwave away inconsistencies. Used in this lazy way retconning is a blight. Used sparingly, it may be essential - or even a storytelling achievement. An apparent retcon is where the person telling the story ALWAYS KNEW THE GIVEN "FACT" WAS INCORRECT and simply withheld the truth as part of the mystery in the story being told.
As well all know, retroactive continuity occurs often in comics. In game terms one of the only relevant effects might be when a previously dead character is discovered to have never died. This is different from a situation where a character, however unbelievably, has "returned from the dead." In the situation where a character believed to have died in fact never did truly die or suffer permanent debilitating injury, then if a hero character was believed to have been responsible, at the gamemaster's sole discretion that character may, when the truth is revealed, receive a special one-off Karma Award or Karma "adjustment" equal to the total Karma lost back when it was believed by all that the character caused the death.
If this became a general trend in the game it would be not only silly but very irritating to keep up to date, but in such a momentous situation as a hero believing (wrongly) that they have taken a life, a restoration of Karma seems appropriate. After all, a person revealed to be alive when thought dead is a pretty big deal itself!