A Retroactive Continuity or Retcon is a piece of new information that imposes a different interpretation on previously described events.
They typically used to facilitate a dramatic plot shift or account for an inconsistency.
With over 40 years of publications and nearly 2,000 publications with the OTU, there are giant inconsistencies within the setting. Many conflicts exist where previous information was overwritten with new sets or otherwise not accounted for when expanding the greater story line/s. Most Traveller versions are relatively stable within their individual version, but have greater conflictual difficulties across versions.
Additionally, while Traveller for the most part avoids Chronotaxis or Time Travel, there are several possible timelines of history. The best known in the GURPS Traveller setting, which is also called the Lorenverse or Imperium Eternal story.
Marc Miller and the Traveller Inner Circle minimally make retcons, but it does become necessary at times to account for the many conflicting details of the setting. Agent of the Imperium, a T5 novel, brought about a whole new set of changes to help transition the new Traveller setting, the Galaxiad.