How often do the networks get outrated?

How often do the four major American networks (NBC, CBS, ABC, and Fox) finish behind cable, syndicated, or public television in the ratings? I do remember a couple of years ago reading an article about an NBA game on NBC being outrated by a wooly mammoth special on Discovery. IIRC, the first run of “The Civil War” on PBS in 1990 and certain Spanish language specials have done the same. Anyone got any particulars?

It’s not unusual for individual network programs to lose to cable programs. I think the only time the four networks combined got a lower total rating than combined cable channels was a period in the summer of 2006.