Counter-intuitives : The 'true' misconceptions

Common-sense occasionally dictates us to behave or react in ways that are scientifically irrational...

#1: When the stock market is on a low, economics dictates that one should 'invest' in the market. Nevertheless, our common-sense (and basic instincts) makes us react otherwise and the 'right way/answer' seems counter-intuitive to ourselves.

#2: Often seen on "Khul Ja Sim Sim" or other Game Shows on TV :

Suppose you're on a game show, and you're given the choice of three doors: Behind one door is a car; behind the others, goats. You pick a door, say No. 1, and the host, who knows what's behind the doors, opens another door, say No. 3, which has a goat. He then says to you, "Do you want to pick door No. 2?" Is it to your advantage to switch your choice?

This paradox, also known as the "Monty Hall Paradox" has its roots in mathematical theory: http://en.wikipedia.org/wiki/Monty_Hall_problem

------

Question -- What other instances of such counter-intuitives have you seen in life ?