Asklemmy

The Toungue map.

The idea that different parts of the tongue are responsible for feeling different tastes. This blatantly false idea was made up in 1901 out of thin air and then made its way into biology classrooms somehow. It was taught to schoolchildren (including me) for about 100 years as a biological fact, even though every human being in that time proved it false by experiment thousands of times by eating things and tasting them with the "wrong" parts of the tongue. It doesn't quite count as an example of this happening today, because we finally realized that it simply wasn't true and have stopped teaching it, but still: 100 years is a long time to realize that something is false when every human being in the world is confronted with physical evidence several times every day.