For me, it was a long talk I had with a random person on Omegle when that was a thing. I was bored one night so I decided to give it a try and I was matched with someone who I had nearly a 2 hour conversation with. We told jokes, told each other about ourselves, and talked like we were lifelong friends. But, we never did tell each other our names. I could’ve talked to this person all night but the interaction turned for the worst near the end. The person was depressed from what I gathered and the depression arose and the conversation fizzled out. I still think about them nearly 6 years later and hope that they are doing good whoever they were.
One time I was biking and someone coming the other way smiled at me. There was nothing to the interaction beyond that few seconds but somehow it was just such a powerful smile it made my whole week and I still remember it.