History is for the most part not a mystery. Chronicles, legends, ruins, artifacts, and other forms of evidence have given us a pretty solid grasp of what happened in previous centuries. We know who fought which battle where and when, who ruled which country, who invented which device, who lived where, and who married which king and when.
And then sometimes we find out that we were wrong.
It isn't a common occurrence: most of our knowledge about the past is based on hard evidence. No amount of scientific innovation is going to change the date of the Battle of Vimy Ridge or the number of people who died in the sinking of the RMS Empress of Ireland
. But some of what we believe to be sound historical fact is based on soft and sometimes unreliable evidence — hearsay, legends, traditions, opinions that have gone unchallenged due only to respect for authority and/or a lack of dissenting voices, reasoning based on data too fragmented to be unambiguous, and occasionally outright forgeries. When new discoveries or new methods of investigation or even new opinions on an event lead to the original belief being discredited among historians, the writer who based his work on contemporary history can be unfairly left looking like he skimped on the research.
As you might guess, the more distant the subject in time the more likely this trope will come into play. We know more about any given day during World War II than we do about the entire reign of the Egyptian pharaoh Khufu, also known as Cheops (the one who built the really big pyramid), for instance.
Compare Science Marches On for when the same thing happens in science.
This trope is not for Alternate History stories where the writer deliberately changes historical fact to explore the possibilities of a new timeline. Examples where a writer simply didn't know better should go in Hollywood History. Examples where a writer deliberately misstates history to make it more palatable go in Politically Correct History. This trope can however be caused by someone in the past misstating history for the purpose of political correctness if their version ends up being taken as fact by later generations. Likewise, this trope is not for cases where an author takes a clear side in something that is currently actively debated by historians, only for situations where later research or revelations definitively reverse the common understanding.
As this reflects changes in understanding of history between the elaboration of the work and later times, any given example can be summarized as a Period Piece, that is also an Unintentional Period Piece on a meta level.
