Continuing my out-of order sequence of Jeff Cooper's 10 rules of firearms safety, I'll jump back to number 1: All guns are always loaded.
Now obviously, all guns are not always loaded, so what's the rule mean? It means we always believe all guns are always loaded and always treat them that way, no exceptions.
In Middle School, I was privileged to attend an Outdoor Education class, an 8th grade 2-quarter elective class taught by Mr. D'Amico. He was an older (in his early 60's?) Italian guy that had more life in him than any other guy his age that I knew. He was an avid outdoorsman, and in his class we learned not just firearms safety (shooting highly accurate pellet guns in the extended classroom), but his knowledge of hunting and fishing.
He had a sobering story about the importance of Rule #1. When he was a teenager, my teacher's oldest brother was proud of his "quick draw" skills. He asked the next youngest brother to go get the revolver out of the gun cabinet so he could practice and show his skill to his family. The oldest brother carefully unloaded the firearm and even had his next-oldest brother verify that yes, it was unloaded, then proceeded to quick draw against his brothers with the unloaded revolver. For some reason or another the oldest brother got called away. He took off the gun belt and set it on the counter. The next oldest brother thought he was all finished, put the cartridges back in the gun and was going to put it away in the gun cabinet when he got distracted by something else. When the older brother came back, he put the gun belt back on, and when the other brother came back into the room, the oldest brother showed him his quick-draw skill one last time, killing him.
Sure there were some odd circumstances leading up to the tragedy, but had rule #1 been followed, the oldest brother wouldn't have the lifelong guilt of knowing he killed one of his brothers.
"I thought it was unloaded" doesn't bring the bullet back when you learn you were wrong. It's a pretty shallow excuse when someone's lying on the floor bleeding to death and you were in control of the firearm.
"I didn't know it was loaded" should be followed with, "so I assumed it was loaded anyway."