Trolling a girl's post one lonely evening, leading to getting married to said girl, in a different country, with a different language. All for the better.
I responded to a flyer on my college campus looking for new players for an upcoming D&D game. 5 years later I got married to the girl who posted that flyer. We're still playing with some of those same players today.
Dating a depressed/suicidal girl for almost 4 years through middle school. To this day she's still the funniest and smartest person I've ever met, but boy did she put me through hell back then.
But I came out of it with an unbelievable amount of patience and mental fortitude. I also learned how to avoid feeding into extreme attention seeking behavior while still having a productive conversation.
Chest cut open for a 2nd time for open heart surgery. Currently on blood thinners because of the mechanical valve they placed in me. A damn growth on one heart valve came back around a half decade after the first time I had my chest cut open and pre-teen me thought the valve was the coolest thing for a short while.
Though, I'm pretty sure this is also the reason my memory sometimes feels off due to how I was under bypass for longer than the surgeons expected during surgery 2.