What facts that 'everyone knows' are in fact, wrong?

I had a (good natured) argument with a colleague the other day when she stated the commonly held belief that 'We only use 10% of our brain" as a means of explaining some of the odder aspects of our world.

I’ve also argued with her when she stated that girls mature faster than boys (only physiologically I said)

Not to pick on her, just that she’s the one I work with most often and talk to the most, there’s plenty of other of these sorts of beliefs I’ve come across elsewhere (and probably hold myself)

Any other examples and are the people promulgating them open to be being corrected, I’ve learned to my cost to be careful how I frame my corrections with some people!

After all nobody likes a ruggedly handsome and devastingly intelligent Irishman correcting them:p

You’re in luck. We had a feeling you’d ask this, and we’ve just finished compiling a whole list of such things for your reading pleasure. Enjoy.

Good question.
Excellent link to similar thread.
Magnifcent closure of thread.