There's a common rumor that there's more people alive now on Earth than have ever lived. I've always found this to be unlikely, but haven't really done any calculations on it. Luckily, some guy has already done the calculations. (See here)
Counting with very low estimates, starting with two homo sapiens 50,000 years ago and using the lower limit of estimated growth and populations, Carl Haub still got the estimation that slightly over 106 billion people had ever been born. The current 6.5 billion is not even close.
Another common rumor is that water running down a drain spins in different directions on the northern and southern hemispheres, but that's not true either. The Coriolis force is just too weak on smaller scales. (See here)
More interesting facts about urban myths and legends can be found at snopes.com