Above is the trailer for The Denial, starring Rachel Weisz as professor Deborah Lipstadt, who was sued in real life by David Irving (Timothy Spall) for the way she characterized his writings on denying the Holocaust. This man, Irving, honestly believed that the Holocaust never happened, and this man was able to mount a lawsuit against Lipstadt because, in the UK, the burden is on the defendant to prove the truth of a statement instead of putting it on the plaintiff to prove that the defendant was being untruthful, as in the case of the United States.
In other words, to succeed in the case, Lipstadt had to prove that the Holocaust did actually exist. One wouldn’t imagine that to be too difficult — she could open a history book, for instance — but the lawsuit took four years to litigate, and the opinion in favor of Lipstadt was 333 pages long. That’s crazy.
The somewhat interesting postscript here is that after Irvin’s appeal was denied, he filed bankruptcy and, several years later, was arrested and sentenced to three years in prison for denying the Holocaust because, in Austria, it is illegal to minimize he atrocities of the Third Reich.