(Disclaimer: I considered not posting this because it is a very sensitive subject, but I also felt it needed to be posted as a way to help getting people who do "believe" in this to finally admit they are wrong)
The Holocaust was the most horrendous acts of mass murder in human history. It killed six million European Jews, and maybe just as many people in other ethnic groups and religions as well. Despite all of the imagines of this horror that has been burned into our minds through photos, videos, testimony from survivors, the soldiers rescued them, and even members of the SS, there are still a group of people who don't believe, or at least they claim they don't believe, the Holocaust happened.
Of all the conspiracy theories there are, Holocaust denial is one of the most disgusting and bigoted conspiracy theories in the world. This conspiracy theory is so offensive, it is illegal to promote in 17 countries, including Germany, and people have gone to prison because they promoted it in those countries.
Not only is Holocaust denial apparently anti-semitic at it's core, it's actually believed by many people who study bigotry, that even people who claim the Holocaust didn't happen, don't even believe it themselves, and that the only reason they deny the Holocaust is to "justify" their own anti-semitic beliefs.
The other "curious" thing about Holocaust denial is, is that it only seems to focus on the Jewish population, and doesn't focus at all on the other groups that were murdered in the Holocaust. This is further evidence that Holocaust denial is purely anti-semitic.
Many historians and scholars consider Holocaust denial so flawed, so biased, and so bigoted, that they refuse to debate Holocaust deniers at all because they don't want to give them a false sense legitimacy.
I'm not sure if this is the most embarrassing conspiracy theory there is, but it's definitely the most disgusting.