The History of the Septuagint

The Masoretic text is what is considered most authoritative within Judaism today. When you go back to the Septuagint it seems to be very different in places. The book of Jeremiah is about one sixth shorter in theSeptuagint which indicates either that the translator was just erasing things left and right or they had a source text that had not yet been added to. There's a theory that the Targumim were actually translated on the fly that's in a service for people who would have stood to read in Hebrew.