Counter Intuitive Math

A one-in-billion event will happen 8 times a month: https://gwern.net/Littlewood

If customers take on average 10 minutes to serve and they arrive randomly at a rate of 5.8 per hour then the waiting time for one teller is five hours while the waiting time for two tellers is 3 minutes: https://www.johndcook.com/blog/2008/10/21/what-happens-when-you-add-a-new-teller/

If you let a 100g strawberry that is 99% water by mass dehydrate such that the water now accounts for 98% of the total mass then its new mass is 50g: https://en.wikipedia.org/wiki/Potato_paradox

https://en.wikipedia.org/wiki/The_Market_for_Lemons

http://en.wikipedia.org/wiki/Winner%27s_curse

Monty Hall problem https://en.wikipedia.org/wiki/Monty_Hall_problem