I debated about publishing this. Maybe to warn others. Maybe to try and clear my quilt.
Here's a picture of my dry fire targets. Notice a problem?

I have a dummy moon clip that I use to practice reloads. it has no primers. Last night I took a break during reloading and thought I'd do a little dry-fire practice and reloads. Grabbed a live moon clip by mistake and took a practice shot at the target. (At least it was an "A". Okay, okay, not funny.)
Fortunately, my dry fire targets back up to a wall which is underground. So no harm done except to my hearing and my ego.
Here's a reminder just above the door on my safe. I put it there when I went on vacation to remind my sons, should they open the safe, that it wasn't.... safe.

My dry fire moon clip looked just like a regular moon clip except no primers, which obviously, I didn't see the difference. Hopefully, this small change will help me in the future.

I didn't really forget rule #1, but I forgot to double/triple check before dry fire practice. Fortunately for me I also have the other rules. Also, I was tired which contributed to not noticing the primers.
Be careful out there and if you forget Rule #1, remember #2 abd #3 and #4.