My clearest personal examples of this are usually like, the first time I was exposed to a sphere of media. I didn't know that I liked CRPGs until I played Planescape: Torment. A bit more nebulously, I'd watched DBZ, Pokemon, Naruto and such like any kid on the schoolyard, but I didn't know I liked "anime" until I watched Gurren Lagann. With the caveat of "I have well and truly abandoned the fandom, but I had fun for a good few years there", Fate/Stay Night was pretty formative; the visual novel's install has gone through three separate PCs.

"Before/After" might not be as much of a hard binary because we did it as a multi-year project, but watching/podcasting through Legend of the Galactic Heroes was a pretty changing experience. It gave me wider references for understanding things in reality.

I didn't know I had ADHD until I saw a PBS documentary about it in a hospital room recovering from an appendectomy. I suppose that counts.

One interesting aspect of this is that sometimes the thing that affects you is like a seed, where you read a thing and without realizing it something is growing inside you and when it finally finishes (possibly years later) you realize how important it was.

For me, a list of stuff that comes to mind (and paints a caricature of me)

• Lord of the Rings, especially the maps
• "The Dispossessed" by Le Guin
• AD&D Dungeon Master's Guide, 1st edition
• "Too Dark Park" by Skinny Puppy
• "The Exorcist"
• "Beyond Good and Evil" by Nietzsche
• Reed Ghazala's writing on circuit-bending
• "Pulse Demon" by Merzbow
• "Mutual Aid" by Kropotkin
• "Making It Explicit" by Brandom
• "The Day the Country Died" by the Subhumans
• "If On a Winter's Night a Traveler" by Calvino
• "Proofs and Refutations" by Lakatos

Probably leaving out a lot of things but those are places where I can remember my life changing in some significant way, or discovering something about myself.

I had this slightly anecdata logic for years that my life was measured by the flipflops I had at the time - as the losing/breaking of a pair was often roughly aligned with new people coming in/out, new environments, etc. Not sure how it works in the current time of multi-pairs - dual wielding destinies I guess

Seeing Hamilton live on stage was one of these moments for me! It felt like it unlocked my ability to cry at emotionally moving art. I don't know how much to attribute to the musical itself; maybe I was already poised to change. But I've cried consistently at every single Hamilton live performance since then (aside from one where obnoxious theater-goers ruined my experience) and wept through a lot more stories, including movies.

• Assassin's Creed really got me to start thinking about philosophies that are more about "tends toward chaos" or "tends toward order" rather than any plain view of good and bad. Some of the greatest wars in real life were not driven by disagreement in the outcome, but the means to achieve that outcome. The writing quality of the franchise really fell off after a while, but it was good while it lasted.
• Story Of Your Life (and its adaptation, Arrival) really showed me two different things at once: the capacity for sci-fi to be deeply intimate, emotional, and human, as well as the idea that in some ways, whether or not free will really exists does not materially matter. We all have a role to play in this world, and a huge bulk of life's journey involves forging or discovering this role and accepting it.
• Annihilation was pivotal. Its visuals invited me to watch it multiple times under different kinds of influence. It very clearly gave us a core cast of characters with differing sets of trauma that form the cornerstone of their being. It also holds a mirror up at the audience and asks: what's your relationship with change itself, which is such a terrifying and unknown thing at times?