The amount and nature of knowledge changes as research on many subjects continue

A few examples

Back when most cars were carbureted people could use premium gas to boost gas mileage and performance. Now with electronically controlled fuel delivery the octane level as little to impact on either in the vast majority of cars.

BMI used to be the gold standard for determining obesity, but it fails to take in to account people that have large frames and a lot of muscle. According to BMI Dwayne "The Rock" Johnson and John Cena are "Morbidly Obese"

The so called Food Pyramid has been debunked in recent years and doctors now recommend diets more based on the specific conditions of the individual in question rather than broad-brushing.

In late 70s experts were warning us that the age of the performance car was over. We now have several cars (including a couple of electrics) than can hit 60 from a standstill in 2 and half seconds. There are now a handful that can reach speeds in excess of 260MPH

IBM executives once told Steve Jobs and Steve Wozniak that no one would ever have a use for personal desktop computers, they of course went on to found Apple.

People used to believe that colds were caused by cold weather because that is when most occurred. The weather itself does not cause more colds, it is the fact that when its cold out people tend to spend more time together in close quarters indoors, those facilitating the spread of the cold virus.

There are still many people who believe that life is a zero sum game, though that has been disproven repeatedly.