I heard or read a story from Freeman Dyson about a somewhat legendary conference where von Neumann was to present his version of Hilbert's problems, presumably about the new computing age, I think at the IAS. It was highly anticipated, but Dyson and everybody else present were shocked because von Neumann was in a state of advanced mental decline when he presented it, and people there agreed to pretend it hadn't happened. Sorry that I can't find a reference to it, the memory of it is vague.