It doesn't happen often but we've seen it before - a broadcaster having a serious health problem as they're live on air. 

The latest case took place at a Tulsa, Oklahoma TV station, where the anchor delivering the news suddenly found herself unable to speak properly. She quickly threw to weather as crews called 911. What's fascinating about what happened is that while she couldn't read the copy, she was able to perfectly ad lib a throw to weather while at the same time explaining her problem to viewers. So the issue seemed to affect her ability to read - not speak. 

You can see it here. 

It has happened at least once in Toronto that I know of. Jerry Agar was in the middle of his show on NT1010 last October, when he suddenly found himself unable to speak properly or complete a sentence. He tried to soldier on, but instead they brought in Ryan Doyle, who was still with the station, to complete his shift. It turned out to be a massive migraine that was so serious, he almost lost the ability to speak. 

Anchor Starts To Have Stroke During Live Newscast