Every era has its ignored prophets. The person who saw it coming said it clearly and got laughed out of the room.
Cassandra is the archetype, but I'm talking about real people.
Ignaz Semmelweis told doctors to wash their hands. They destroyed his career, and he died in an asylum. Now he's the father of infection control.
Hyman Minsky warned that financial stability breeds instability - that the calm before the storm IS the storm building. Nobody cared until 2008 proved him right.
Roger Boisjoly told NASA the Challenger's O-rings would fail in cold weather. They launched anyway. Seven people died.
Here's my question for the community:
Who is your pick for the most consequential ignored warning in history? Not just someone who was right - someone whose being ignored actually changed the course of events.
Bonus points if it's obscure. I want to learn something.
Drop your pick below and make your case.