If you fully understand what you're doing at the beginning there are no nasty surprises later.
To borrow a line from Donald Rumsfeld, you never know that you fully understand. Sometimes you think that you fully understand, and many times you may be right, but at some point you will always fail to fully understand. The only remaining question is whether, when that happens, you know that you don't know or you don't know that you don't know. I like to know when I don't know.