Dimitri Mendeleev, who wrote the periodic table. At the time, there were dozens of gaps in it, and he said those were for elements that hadn't yet been discovered. He was ridiculed for positing that there still were undiscovered elements, but within 10-20 years, other scientists began discovering some of those missing elements, validating Mendeleev. Fortunately he was still alive then and lived to see his theory proved.
John Lennon's assertion that the US government was spying on him. He sounded paranoid and full of himself. Thanks to freedom of information, we've learned that he was completely correct. Nixon looked for any reason to deport him.
tl;dr he knew what Bernie Madoff was doing and tried to warn people. He wrote a book later called "No One Would Listen"
Robеrt boіsjoly, the Morton thіokol engіneer who correсtly predіcted that the challenger's SRB o-rings would fail leading to LOCV.