Exactly when it happened cannot accurately be determined. It could have been made by early copyists since some existent manuscripts date back to the fourth century A.D., near the time of the Council of Nice. It is unlikely to have happened much later. Later copyists are known to have made errors and interpolated notes into the text. However, the early manuscripts would probably exonerate them from having made the change.
Fault can lie with modern preparers of Greek texts and with modern translators.
If a few translators have caught the discrepancy, some others should have also, especially since some versions use cross-references which link these passages. Based on the many problems that translators have and their willingness to admit that their work is not perfect, my charge may be too harsh. Let us look at what the New Testament passages say. That everyone in the world will mourn when the Messiah returns to the earth is a statement which seems to be quite possible. However, for everyone in the world to see Him coming on the clouds, it would be necessary for Him to create a miracle in defiance of His laws of physics and astronomy.