And amateur astronomers of America (plus Russian professionals) claims of seeing flashes on the Moon in the 1950s and 60s were dismissed,everyone knew the Moon was unchanging and all the craters were millions of years old!!!It was settled science.
Comet Shoemaker–Levy collided with Jupiter in 1994. The documentary that I watched said that the impact ‘proved’ that impact with the Earth was possible. Going with that comment, I assumed that as late as 1994, there was still some scientists who doubted that.