It was either finding out that the idea what I came to at about 7 regarding galaxy formation then discarded as unlikely was demonstrated probably correct by Hubble in my late teens...
No, it's this.
I was at a convention. My first ever convention, and so far still my only convention. Ursula K. LeGuin was making a rare public appearance, reading from a new book and entering into lively conversation, and Peter S. Beagle was selling and signing books and later would sing a few songs for us. (Damn it, there's more, but it's not relevant. Gasm.)
I had been thinking about mythic tropes in his book The Last Unicorn. I'd also recently read his novella Two Hearts, a sequel to the aforementioned book, and was surprised (though not shocked) to have my idea of the mythic nature of their story arcs bear fruit in the novella as well. I brought up this trope with Mr. Beagle, and he identified the trope by name (he referred to it, I believe, as "the summer king" trope), and said that he most likely had been familiar with that mythological concept when he wrote it but was fairly certain he did not have it in mind. However, he liked that it did, in fact, fit both stories, and apologized in advance that he might forget he didn't come up with it himself.
I got light-headed. Not dizzy, so not literally a "nerdgasm," but I had to go lay down.
If it has to be video game related, it would probably be seeing the storyline play out for Xenogears. Frustrating presentation, but staggeringly good story.
