During the early pandemic, he underestimated the rate of fatality from COVID (which, remember, was much higher before vaccines and paxlovid were deployed), and forcefully advocated for policy based on his lower fatality rate estimates. It was a stunning display of hubris: working on very limited information, he was pushing incautious policy responses which could have cost millions of lives.
https://en.m.wikipedia.org/wiki/John_Ioannidis