I was aged about 9 and I very nearly caused another child ( same age) to drown.
School swimming lessons in an outside pool belonging to the local Grammar School. We got sorted into groups according to swimming ability. The non-swimmers were down at the shallow end and the rest of us were in different areas of the pool. I was a good swimmer.
Cue a small break in the 'lessons' near the end of our time there. I went up to one of the higher diving boards and another girl was standing on the edge of the board looking down. She kept saying she couldn't decide whether to jump in or not. She was faffing around and I got really impatient in the end and pushed her in - and jumped in after her.
The whistle went as I hit the water, so I swam over to the side and got out. Standing on the side while our teacher gave us a talk, I turned around and this girl was still in the water and her face was turning black. I didn't know what to do, but fortunately someone else noticed, dived in and rescued her.
Turned out she COULDN'T swim, was in the non-swimmers group and what on earth she was doing on the high diving board and saying she was trying to decide whether to jump in I still do not know.
I have never forgotten that and often think about what could have happened. That was a lesson to me not to take anything anyone says at face value, without considering things. It was a terrible thing to have nearly happened, even if the blame wasn't all entirely mine. I still get hot and cold when I think about it