Here is the 50 - 90 rule: If there is a 50/50 chance of doing a job right, there is a 90% chance it will get done wrong.
For example, the hoses on the back of my washing machine. Only two ways to put them on. Right, and backwards. Of coure, the next morning when the water is warmed back up, I find out that they are hooked up backwards.
See?