Some obvious things that demonstrate human capacity for denial
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Lead is poison
It's right there in Boook VIII of Vitruvius's De Architectura:
"Water conducted through earthen pipes is more wholesome than that through lead; indeed that conveyed in lead must be injurious"
Note, this was in 15BCE. Vitruvius wasn't alone in thinking this--ancient physicians actually included lead on their lists of poisons. Despite realizing very early on that this stuff was poisonous, the Romans continued using it to make water pipes. While modern readers like to make fun of the ancient Romans for this hypocrisy, in fact modernity has been far worse on this issue. Tobacco is poison
Look, it took King James I all of two seconds to recognize that this newly discovered plant was very bad for you. He discouraged it's use and levied the first Pigou tax ever--6 shillings 10 pence per pound of tobacco--in 1604, arguing his position in a famous pamphlet entitled "A Counterblaste to Tobacco:"
"Have you not reason then to bee ashamed, and to forbeare this filthie noveltie, so basely grounded, so foolishly received and so grossely mistaken in the right use thereof? In your abuse thereof sinning against God, harming your selves both in persons and goods, and raking also thereby the markes and notes of vanitie upon you: by the custome thereof making your selves to be wondered at by all forraine civil Nations, and by all strangers that come among you, to be scorned and contemned. A custome lothsome to the eye, hatefull to the Nose, harmefull to the braine, dangerous to the Lungs, and in the blacke stinking fume thereof, neerest resembling the horrible Stigian smoke of the pit that is bottomelesse."[emphasis added]
By the way, if you recognize that byline, it could be because you've read his other famous publication, the Bible. The King's suspicions have been overwhelmingly confirmed by modern science--and is in fact far worse than he knew--yet tobacco remains one of the leading causes of death in the US.Perfluorooctanoic acid is poison
Huffington Post has a long-form piece about the DuPont corporation's 50-year conspiracy to cover-up the acute toxic effects of the chemical behind it's star product Teflon and the West Virginia town it devastated. Despite knowing as early as the 1950s that the chemical was extremely dangerous, DuPont suppressed results and falsified lab tests while illegally dumping the waste in public waterways, only recently phasing out use of the chemical in 2014. For what it's worth, they hired the same PR hacks as the tobacco industry. But on the bright side, DuPont has switched to a series of very similar compounds whose health effects are unstudied, but likely to be about as bad.
Carbon emissions are poison
Ok, not poison in the medical definition per se. But it's clear that carbon emissions, resulting from human activities, are causing global temperatures to rise with very serious, possibly catastrophic, effects. Yet despite clear science and broad consensus, the entire GOP presidential field falls somewhere on the denial spectrum.
Football causes brain injury
The NFL has only recently accepted that this is true, claiming that until last year no evidence of brain injuries in football ever existed. Frankly, the NFL's position is absurd. Despite the NFL's 20-year conspiracy to obfuscate the facts, the proposition that football causes serious brain injury has simply never been controversial.
This 303 pound tumor
I saw this case study as an undergrad and it has left an impression on me ever since. Any doctor would have recommended surgery for any tumor even a fraction of this size, suggesting that this person was in such deep denial that she refused to see a doctor, or refused the doctor's advice. When it comes to any of the issues above and many others, we are all this woman.