In an erratum to my previous post on Pascalian wagers, it has been plausibly argued to me that all the roads to nuclear weapons, including plutonium production from U-238, may have bottlenecked through the presence of significant amounts of Earthly U235 (apparently even the giant heap of unrefined uranium bricks in Chicago Pile 1 was, functionally, empty space with a scattering of U235 dust).  If this is the case then Fermi's estimate of a "ten percent" probability of nuclear weapons may have actually been justifiable because nuclear weapons were almost impossible (at least without particle accelerators) - though it's not totally clear to me why "10%" instead of "2%" or "50%" but then I'm not Fermi.

We're all familiar with examples of correct scientific skepticism, such as about Uri Geller and hydrino theory.  We also know many famous examples of scientists just completely making up their pessimism, for example about the impossibility of human heavier-than-air flight.  Before this occasion I could only think offhand of one other famous example of erroneous scientific pessimism that was not in defiance of the default extrapolation of existing models, namely Lord Kelvin's careful estimate from multiple sources that the Sun was around sixty million years of age.  This was wrong, but because of new physics - though you could make a case that new physics might well be expected in this case - and there was some degree of contrary evidence from geology, as I understand it - and that's not exactly the same as technological skepticism - but still.  Where there are sort of two, there may be more.  Can anyone name a third example of erroneous scientific pessimism whose error was, to the same degree, not something a smarter scientist could've seen coming?

I ask this with some degree of trepidation, since by most standards of reasoning essentially anything is "justifiable" if you try hard enough to find excuses and then not question them further, so I'll phrase it more carefully this way:  I am looking for a case of erroneous scientific pessimism, preferably about technological impossibility or extreme difficulty, where it seems clear that the inverse case for possibility would've been weaker if carried out strictly with contemporary knowledge, after exploring points and counterpoints.  (So that relaxed standards for "justifiability" will just produce even more justifiable cases for the technological possibility.)  We probably should also not accept as "erroneous" any prediction of technological impossibility where it required more than, say, seventy years to get the technology.