I have been using this line a lot lately. It is a bastardized version of Field Marshall Helmuth von Moltke's "No plan of operations extends with certainty beyond the first encounter with the enemy's main strength."
I always encourage entrepreneurs to get on with the business of putting their product in the market. All the planning and designing and strategizing and grand plans of taking over the world are no match for reality.
The real world is messy. Stuff happens that you could never imagine. And then you are reacting to all of that and your grand plans are in tatters. That's reality. It happens to everyone.
So no point in waiting in the hopes that you will nail it. You won't. The enemy will take fire, you will hit the deck and then the good stuff starts.