Had a moment like that Sunday. I dropped my mag full of defense rounds and put in the FMJ magazine. I set my targets up and got ready to empty a mag or two plinking at cans before I ran my drills. That breaks the monotony and puts a smile on my face. I started doing this because it was fun, anyway. So I unholstered, racked the slide, on target...wait, what was that? Yep, I forgot to empty the one in the chamber. I wasn't expecting to see it go flying when I racked it. No big deal, but my carelessness startled me since I just knew the chamber was empty. I observed all four rules, but I was just positive about the condition.
Reminded me that the rules are there for reason.