Worst for me was a Foreigner concert with April Wine as the opener. Apparently at the time, 1980 or so, April Wine really did a stand up OVER Foreigner so they, Foreigner didn't want April Wine to play before them in the Pocatello, ID Mini-Dome.
Needless to say, the concert was 1.5 hours LATE starting, and the PA portion of the sound system did not work. Band (Foreigner) played great, sound sucked very bad, including Urgent, and overall, it was like listening through a pocket AM radio.
No encore either...