"It ain't what you don't know that gets you into trouble.  It's what you know for sure that just ain't so."
-- Mark Twain

As humans, we tend to have a very high opinion of ourselves, particularly when it comes to overestimating our skills and abilities.

This is abundantly apparent in our attempts to predict the future.  It's a particularly dangerous exercise when it comes to investing because not only are we woefully bad at predicting outcomes, but even when we get it right, it only "counts" if we were right and the market consensus was wrong (if we know something that everybody else knows, it's not terribly useful).

How much do you think you know about the future?
Cut it in half.  Ten times.  Now you may be close.

"Since no man knows the future, who can tell him what is to come?"
-- Ecclesiastes 8:7

"The race is not to the swift or the battle to the strong, nor does food come to the wise, or wealth to the brilliant, or favor to the learned, but time and chance happen to them all."
-- Ecclesiastes 9:11

"Cast but a glance at riches and they are gone, for they will surely sprout wings and fly off to the sky like an eagle."
-- Proverbs 23:5

Build an investment plan based on an unknowable future. 
Have a healthy respect for uncertainty.