Statistical significance & other A/B test pitfalls

Cennydd Bowles with words of caution about A/B test results. Great intro:

Last week I tossed a coin a hundred times. 49 heads. Then I changed into a red t-shirt and tossed the same coin another hundred times. 51 heads. From this, I conclude that wearing a red shirt gives a 4.1% increase in conversion in throwing heads.