Common Misconception: "8 inch per mile squared"
 
One thing you hear flat earthers say often is that the curvature of the earth is 8 inch per mile squared. 

This is only a rough estimate for the first few miles though.  

It is way better to use the Pythagoric Proof: http://imgur.com/a/vI4XF





Original autocad drawing made by  M. Kahnke (07/02/15)