Frank L. H. Wolfs
Department of Physics and Astronomy, University of Rochester, Slide 52
The prediction of black holes.
¥Einstein tried to modify his theory by adding a constant such that it would not predict the existence of black holes.  This constant became known as the cosmological constant and accounted for all missing physics in the theory.
¥Eventually Einstein gave up; there appeared to be no need for the cosmological constant, and Einstein called it "his greatest blunder".