Albert Einstein was furious with himself for introducing the cosmological constant into his general relativity theory and thus failing to predict that the universe is expanding. This failure was certainly not his greatest mistake, as he said. Austrian physicist Friedrich Hasenorhl published the equation E=mc2 a year before Einstein did, yet Hasenorhl didn't connect the equation with the principle of relativity. Einstein's great mistake was being unaware of something associated with his theory of Special Relativity which would have clarified his view of quantum theory and undoubtedly led to his successfully finding the unified field theory of the universe.
In special relativity, he concluded that nothing travels faster than light, but space itself has no such speed limit (immediately after the Big Bang, the runaway expansion of the universe apparently left light lagging way behind). Common sense and logic dictate that light and space must be different and separate since the velocity of one is different from the rate of expansion of the other. However, the universe is stranger than we can imagine. So, in keeping with the scientific intuitive leaps which are necessary to approach an accurate description of the universe and its workings, Einstein's curiosity and imagination could (except for his unawareness) have taken an indirect, 2 step, path to this logical deduction:
1st, by considering the opposite: that quantum particles like the photons which compose light are not separate from space itself. The universe would not merely be a vast collection of the countless photons, electrons and other quantum particles within it, but would be a unified whole that has particles and waves built into it, just as a computerised hologram would have seemingly separate points built into its union of digital zeros and ones. 2nd, it would be necessary to suggest how this "illogical" unified whole could appear to us as an infinity of different and separate entities (not only in space but also in time). If light and space were unified in a digitised hologram, the apparent velocities of light and space's expansion would differ because light would be a subroutine embedded in the main space program.
Such an insight on Einstein's part would have resulted in reconciliation of relativity with quantum physics' entanglement, quantum tunnelling, instantaneous interactions and (to use a phrase he coined) "spooky action at a distance". I believe he was perfectly capable of such intuitive leaps. He had all the required pieces for constructing a new cosmological jigsaw ... a "unified field" model of the cosmos. It had been some 20 years since the astronomer Edwin Hubble showed that the universe is expanding. He was familiar with the philosophical notion of a static universe, into which he could have integrated Hubble's universe by showing that the expansion is only apparent and actually exists as a subroutine in a static (or steady state) hologram - this would have justified the cosmological constant in his own lifetime, too. By the mid-1940s mechanical computers were replaced with electronic ones. In 1947, physicist Dennis Gabor invented holography. So what accounts for Einstein's lack of vision? Maybe old age had simply caught up with him (he was, after all, 70 years of age in 1949) . Maybe "Uncle Albert" had become too conservative and fixed in his thinking.