I've got a couple.
Several years ago, I was out on my boat to dive with a friend of mine. Planned to be a spearfishing dive. My friend was new to spearfishing and hadn't purchased his own speargun yet, so he was using my spare. We arrived at the spot, and started gearing up. That's when I realized that I screwed up. Had mask, fins, BC, reg, weights, computer, camera, gloves, knife, speargun. Ready to go except I had left the spears at my house. Oh well, I guess this is going to be a non-spearing dive. It wasn't a total loss. My buddy found a titanium knife on that dive.
On another occasion, spearfishing with a friend. Backroll in, and arrive at the bottom. when on the bottom, I notice that someone had dropped a weight. I pick up the weight. Before I could place in my pocket, I see another one. At that point, I realize that they are mine. Turns out that when I added the weight, I forgot to zip the top of the pocket closed. New BC, so I wasn't as used to it then as I am now. Luckily, it happened at the beginning of the dive so I still had a full tank of air to keep me negative. Trim weights were secure, just not the main, so I think it was just a total of 4 lbs.