猿も木から落ちる (さる も き から おちる）
You can have done something a thousand times. Easily mastering it with your eyes closed, hands behind back, spinning plates.
Go to impress someone with it and mess it up so royally you have to move town, change your name, burn your finger prints off with battery acid.
Don’t Worry, as the old saying goes “even monkeys fall from trees”