I almost made a mistake that even a beginner wouldn't do.
I was leaving a machine with credits still remaining.
In retrospect, there is a situation when this mistake might be more common than you'd expect. It occurs when you've depleted your credit balance to less than the maximum bet for another hand. In my case, I was playing $1 Five Play. I didn't intend to remain and insert more into the machine if I lost what I originally brought.
So when the balance got down to $15, I simply got up and started to walk away. My mind was clearly on my next move and where to play next.
Luckily, I was only a machine or two down the row when I glanced back and realized my mistake.
I mention this experience because on that day I sat next to two different players who actually DID walk away with credits remaining. In both circumstances the balance was smaller than the max bet for another hand.
And in case you're wondering, yes, I did quickly alert both players as to their mistake before they wandered too far.