Yep. Here's an easy-looking one, that lasted just under 2 centuries (quoting Wikipedia) -
> In number theory, Euler's conjecture is a disproved conjecture related to Fermat's Last Theorem. It was proposed by Leonhard Euler in 1769. It states that for all integers n and k greater than 1, if the sum of n many kth powers of positive integers is itself a kth power, then n is greater than or equal to k...
> ...
> Euler's conjecture was disproven by L. J. Lander and T. R. Parkin in 1966 when, through a direct computer search on a CDC 6600, they found a counterexample for k = 5.[3] This was published in a paper comprising just two sentences.[3]
> [3] - Lander, L. J.; Parkin, T. R. (1966). "Counterexample to Euler's conjecture on sums of like powers". Bull. Amer. Math. Soc. ...
What exactly are you saying this is an example of?
It's certainly not something that people believed and built stuff on the basis of; it was never regarded as anything more than a conjecture and I would be a little surprised if even one paper was published that took the conjecture as a hypothesis, even explicitly (i.e., "We show that if Euler's conjecture is true then ...").
It's also not, so far as I know, a case where anyone reacted with defensiveness, horror, insecurity, etc., when a counterexample was found. They published a paper in a reputable journal. They don't seem to have had much trouble getting it published, if they discovered the counterexample in 1966 and the paper was published in a 1966 issue of said journal.
So if you're suggesting that this is a case where "people were building on swathes of mathematics that seem proven and make intuitive sense, but needed formal buttressing", I'd like to see some evidence. Same if you're suggesting that this is a case where "so-called experts had invested far, far too many of their man-years in that unproven conjecture" and there'd be a hostile reaction to a counterexample.
On the other hand, if you're not suggesting either of those things, I'm not sure what the connection to the rest of the discussion is.
> What exactly are you saying this is an example of?
A prominent conjecture in number theory, taken quite seriously for centuries, but which was quickly and rather easily disproven once computers became powerful enough.
No, it is not a exact analogy for Fermat, nor BSD, nor Riemann, nor ...
My initial point of interest was u/bootby's comment - why the heck would a room full of experts (presumably noteworthy math professors) become so angry at some grad student's comment? Then /usr/baruz's comment, about things which "seem proven and make intuitive sense, but needed formal buttressing". On occasion, "seemed" and "intuition" prove to be wrong, and Euler was a pretty-good example that.
and a famous conjecture is by definition something for which all the experts know that its truth is UNKNOWN (even in cases where most experts believe it's true).
> In number theory, Euler's conjecture is a disproved conjecture related to Fermat's Last Theorem. It was proposed by Leonhard Euler in 1769. It states that for all integers n and k greater than 1, if the sum of n many kth powers of positive integers is itself a kth power, then n is greater than or equal to k...
> ...
> Euler's conjecture was disproven by L. J. Lander and T. R. Parkin in 1966 when, through a direct computer search on a CDC 6600, they found a counterexample for k = 5.[3] This was published in a paper comprising just two sentences.[3]
> [3] - Lander, L. J.; Parkin, T. R. (1966). "Counterexample to Euler's conjecture on sums of like powers". Bull. Amer. Math. Soc. ...