I know there are better examples, but the one that always comes to mind for me is Asbestos. Everyone thought that was safe, now it kills 100,000+ people a year.

And to make matters worse, companies were able to suppress evidence of its danger for decades. We never know what evidence is being withheld from us, so I feel like a little irrational paranoia is actually rational.

Edit: Oh, and how about Exxon withholding all that information about climate change for 40 years? That's another good one.