Probably the most famous anchor swap I can recall, though happened off-camera, was when the late Jill Singer collapsed during a commercial break on Today Tonight in Melbourne after having to announce that the show had to drop a major story about the Kennett state government. Story goes that she was visibly stressed when making the announcement and that she fainted in the ad break so newsreader David Johnston was hurriedly called back to set and, as he described it at the time, literally had to step over her lying on the floor to get to the seat at the desk. He then went on to present the rest of the program.
Ms Singer recovered from that episode but left the show soon after.