Online opt-in polls can produce misleading results, especially for young people and Hispanic adults
www.pewresearch.org
We examine how an opt-in poll may have unintentionally misled the public about the sensitive issue of Holocaust denial among young Americans.

There was an opt-in survey in from YouGov & The Economist that seemed to show a fifth of young Americans denied the Holocaust. The Economist ran a piece on it.

Well, Pew Research did a random sampling and found completely different and far less worrying results.