I *went* for King Crimson in 1974, but the opening act was Steeleye Span (who were a lot of fun). Nearly half the audience was there for them and left at half time, not bothering to stay for KC!
I also just remembered that when I saw Soft Machine in 74 (at the same venue - Kennedy Ctr), Softs were the opening act for Eleventh House, who I was not there for....
As far as unknown acts blowing the established acts off the stage, one of the most profound for me was Etron Fou just destroying Henry Cow. Twice.
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