What are the regrettable things in the history of mathematics?

Hilbert has another regret, that is, he wants to see the Riemann conjecture proved in his lifetime. It is said that one of his students wrote a paper to prove Riemann's conjecture. Although there are mistakes in this, Hilbert feels that this student has pointed out the direction for him, so he wants to continue the discussion with this student. However, a bolt from the blue, the student died the next year, I don't know what happened. History is always so ruthless, it should be that death is always so ruthless. Either mathematicians have no paper, or mathematicians fight, or mathematicians have an accident, or his students have an accident ... Now the normal mathematician is probably Tao.