Steven Weinberg in Physics Today:
Albert Einstein was certainly the greatest physicist of the 20th century, and one of the greatest scientists of all time. It may seem presumptuous to talk of mistakes made by such a towering figure, especially in the centenary of his annus mirabilis. But the mistakes made by leading scientists often provide a better insight into the spirit and presuppositions of their times than do their successes.1 Also, for those of us who have made our share of scientific errors, it is mildly consoling to note that even Einstein made mistakes. Perhaps most important, by showing that we are aware of mistakes made by even the greatest scientists, we set a good example to those who follow other supposed paths to truth. We recognize that our most important scientific forerunners were not prophets whose writings must be studied as infallible guides—they were simply great men and women who prepared the ground for the better understandings we have now achieved.
The cosmological constant
In thinking of Einstein’s mistakes, one immediately recalls what Einstein (in a conversation with George Gamow2) called the biggest blunder he had made in his life: the introduction of the cosmological constant.
More here.