I can't think of any particular issues that I'm convinced I know the truth of, yet most people will reflexively deny that truth completely.

I can, however, think of issues that I think are uncertain, but that the uncertainty of said issue is denied reflexively and completely. I suppose they would be meta-issues rather than issues themselves - it's a subtle point I'm not interested in pursuing.

Probably the most obvious one that comes to my mind is circumcision. I've never seen so many normally-intelligent people make such stupid and clearly incorrect arguments, nor so much uncomfortable humor, nor trying desperately to avoid thinking, for any other issue I've discussed with others, even things like abortion, religion, and politics.