In her infamous 1993 book Denying the Holocaust: The Growing Assault on Truth and Memory, Prof. Deborah Lipstadt denies that there were homicidal gas chambers in concentration camps on German soil; denying the undeniable spherical Earth-esque truths that there were homicidal gas chambers at Neuengamme, Ravensbrueck, Sachsenhausen, and Dachau*:
"... it was precisely those whom [revisionist Harry Elmer] Barnes accused of being "court historians" who, in fact, were responsible for demonstrating that there had been no homicidal gas chambers in German concentration camps. After the war there had been persistent confusion about the difference between concentration camps and death camps. The latter, located outside Germany, had facilities for the express purpose of murdering people, primarily Jews. While there were no death camps in Germany, there were many concentration camps, in which multitudes died from overwork, disease, starvation, beatings, and severe mistreatment. Much of the confusion centered around the idea that there was a functioning homicidal gas chamber in Dachau. This was what historians were trying to clarify in 1962, when Professor Martin Broszat, who served for many years as the director of Munich's Institute for Contemporary History, wrote to the newspaper Die Zeit to "hammer home, once more, the persistently ignored or denied difference between concentration and extermination camps." Contrary to deniers' claims, he said, his letter did not constitute an "admission" on his part but an effort to "set the record straight." This remains a consistent tactic of the deniers. Every time historians who study the Holocaust correct a mistake in the record, deniers immediately claim that they do so because their previous lies were about to be exposed."
* Dachau is the "special case", and to be fair to Lipstadt, when she wrote her book the Dachau museum claimed its gas chamber had never been used. That fact was jettisoned in 2003 and replaced by the 'it was used experimentally' fact. Visitors to the 'gas chamber' are deliberately given the impression that it was often used, and it has been proved that the Dachau museum decided against informing visitors in the transparent manner recommended to them by the House of Bavarian History, that the 'gas chamber' is not claimed to have been used for mass-gassings.