WHile driving, I saw a guy on a bike with no lights. I saw him in lots of time and distance, and was able to go round him with no issues. Admitedly this was on a wide, well-lit, urban road.
Stopped at the lights on my bike, I looked behind me and noticed that the driver of the car behind hadn't turned their lights on. So I made flashing gestures at them until they realised what I was trying to communicate to them, and turned the lights on and gave me a wave of thanks.
But the interesting one was this morning. Someone got off the train in front of me with a red flashing rear light. Close up it looked quite weedy and I decided that there wasn't any point in putting my lights on as it was pretty light, and I was wearing a high-vis tabard. "If they can't see the high-vis, they're not going to notice me full stop" was my logic. However, as the other cyclist pulled away I realised that their rear light did make a difference from a distance, and I wished I'd put mine on too. (Not that I actually stopped to put them on!)