Halloween night 1970: University of Miami soccer field.  Black Sabbath in concert as part of what may have been their first U.S. tour.  Much of the crowd was dressed for the occasion.  There was no opening group.  Black Sabbath appears more or less on time.  Power to the stage fails in the middle of the first tune.  UM crew fixes it.  Power fails a second time in the middle of the first tune.  UM crew fixes it.  Sabbath makes it through an entire tune, I think it was 'Paranoid'. Middle of tune two, another power failure.  Ozzy throws the mic down, curses, and walks off, followed by the rest of the group.

Considering I had only driven 3 miles and somehow got in(I still had my UM student activity card), I wasn't too put out.  Next day, went out, bought the first album (had only heard Paranoid before), and wore it out listening to and learning the tunes.  

I saw the group at least 3 times in successive years, all good concerts.