We had a similiar thread in /r/badhistory http://www.reddit.com/r/badhistory/comments/2pi0g9/what_are_some_good_examples_of_bad_history/

The elders of Zion hoax and Dolchstoßlegende are very on top of bad history having very bad effects.

The Lost Cause of the South would probably be the greatest example in contemporary American politics, indeed it is still widely believed in and espoused - for example, libertarian critiques of Lincoln "interfering with property rights" or the annoyingly persistent myth that it "wasn't really about slavery".

The "clean Wehrmacht" myth has proved annoyingly durable too; and it was used during the Cold War to diminish and discredit the massive Soviet contribution to WWII. See this book: http://www.cambridge.org/us/academic/subjects/history/american-history-after-1945/myth-eastern-front-nazi-soviet-war-american-popular-culture

Outside of that...there are things like the stab in the back myth, but that was more a conspiracy theory that history for those who believed in it.

Sorry, we don't allow throughout history questions. These tend to produce threads which are collections of trivia, not the in-depth discussions about a particular topic we're looking for. If you have a specific question about a historical event or period or person, please feel free to re-compose your question and submit it again. Alternatively, questions of this type can be directed to more appropriate subreddits, such as /r/history or /r/askhistory.

Might be a questio to pose in /r/BadHistory. "Examples of badhistory influencing real stuff?"