Politics is the math-killer.

I noticed it for the first time when I was at university. There was a political discussion about impacts of privatizing, and some guy said: "The prices of electricity have increased by 10%, and the prices of heat have increased by 10%, and the prices of water have increased by 10%... which means that for people the total costs of living have increased by 30%..." and many students were nodding in agreement, and I was like: WTF is happening here? Here is the best mathematical university in our country, and people are seriously believing that if you split something into small parts and increase three of them by 10%, then the whole has increased by 30%? How is that even possible? I was already aware that politics can make people crazy, but I didn't imagine it could ruin their mathematical skills so completely. It took some time to explain, and even then instead of admitting an error some people offered some rationalization instead (such as: the 10% was the estimate for now and the 30% was the extrapolation for the future; or that they meant "more than 10%" and "almost 30%", which is not completely exclusive; etc.). At that time I already knew that most people hate to admit they made an error.