For me, the most disappointing concert that I caught in all my years of show attending was Van Halen on the 1984 tour.
I am sure that the internal battles the band was going through played a big part, but Eddie, though not lacking in any way with his guitar work, did not seem to be playing with the rest of the band. He seemed to be on one long extended practice and paid no attention to anyone on stage.
Dave was just wasted, singing out of time and out of tune; and forgetting lyrics even, going off a number of times on some scat sing along with all that be-bah-bah-bah type stuff.
I left shocked and saddened as the tour previous that I caught, Fair Warning in 1981, was a total killer time.