Make no mistake.
That's it. I'm not going to follow that up with some bullshit like most people who use the phrase do.
Discussion
The only one mistake free is Jesus Christ
Think of the phrase like a wish or a charm, akin to "go kick some ass" or "fare thee well".
Of course we will make mistakes, and we will not always fare well. But it's nice to be well-wished.
Let's be clear.
That's it.
People using such phases tells you everything you need to know..
... Sorry I can't pretend. It doesn't tell me enough. I want to know their hobbies, hopes, and dreams.
Verily I say unto thee...
*insert tired recycled meme*
do you understand now ?