After reading one of my posts on the differences between the learning processes of males a females a commenter noted that it was "common knowledge" that brains whether they be male or female basically function the same.
So, I started thinking about other things that are "common knowledge" that are also completely wrong.
Julius Caesar was emperor of Rome.
In Caeser's time, Rome was a republic and had no emperor.
Diamonds are the most valuable gems.
Carat for carat, rubies are more valuable.
Ticks are insects.
Ticks have 8 legs. That makes them arachnids.
Chameleons change color to match their background.
Chameleons change color in reaction to fear, not to match their background.
Arabic numbers come from Arabia.
The numeric system we use originated in India, not Arabia.