Local oddball music promoters "Howlback Hum" recently celebrated their 10th Birthday by putting on 6 gigs in 5 days, in 7 different venues (?).  Or rather, they INTENDED to do that. But upon setting up their cobbled together PA system each night, they found that they were getting uncontrollable ear-splitting howlback from the microphones, and bowl-shaking hum through the PA speakers.
So, it was business as usual, hehe, and the gigs went on without a hitch!