Well, here's something you don't see (or hear) very often on a usually well rehearsed and co-ordinated nightly network newscast - a fire alarm going off right off the top of the show. 

It happened on Friday's ABC World News Tonight, just as anchor David Muir came on after the opening teasers at 6:32 PM. I'm guessing there was nothing they could do to move elsewhere or stop it, so it sounded even as Muir calmly greeted viewers. 

"We should note that the alarm just started going off here in our studio just as we came on tonight," he told viewers as the show began. "We will let you know if indeed it is a real alarm in a moment here, but I'm going to try and push through the news here in the meantime." 

He then threw to the story about Hurricane Isaias about to rampage through several southern U.S. states. 

The noise continued through the next intro as well, but was gone by 6:40 PM and the newscast continued as normal. 

Muir never did tell viewers what set it off or if the situation was resolved. 

I only remember one time it happened at the newsroom where I was working at the time. But unlike ABC, ours was rigged to sound an alarm outside the studio area, while some strobe lights flashed off and on to alert staffers something was up. That also turned out to be a false alarm.