I was the main tank of a raiding guild in WoW during the Burning Crusade era. Our guild was the best on the server, but nothing too notable outside of that community. However, when a new quick raid encounter came out (Magtheridon's Lair), we unintentionally completed the encounter in a novel way. I know that because the next week when we went to clear it, a developer whispered me and asked me if he and a few others could watch us because they noticed through some automated log to prevent cheating that we completed the encounter last week without engaging with a major mechanic of the fight. They let us know they were there, but you couldn't see anyone and when you looked up the character name it wouldn't show up if you searched the name. We showed them how we did it, they congratulated us for our ingenuity then told us they were changing the encounter for next week. It was really cool.
A few years ago I got into Enter the Gungeon. For those unaware it's a bullet hell, rogue-like, dungeon crawler with a steep learning curve but great graphics and snappy controls so it is a great time. The first time I beat the game I posted a screenshot on Reddit and the entire community was convinced that it was fake because of my build and the lack of max health I had. At first I was annoyed then I realized that if they didn't believe me then it meant I did something, literally, incredible.
100%'d blast corps back in the day, caught the hylian loach before there were any guides about it in ocarina of time.
And more (but not terribly) recently, I completed an 'all bosses' challenge run in terraria in which I made all naturally spawning blocks indestructible.
Me and all members of my clan (9 players) in the top20 players in battlefield 1942 desert combat mod. We were from a stupid small village, lived and breathe that game.
And for cs:source i was number one scout player for a while. Always top 5.
Managed to reach a rank of 2450 in World of Warcraft 2v2 arena. It took an incredible amount of much time and effort, and I have accepted that I will never be as good at anything ever again.
Was the first one dropping the Cloak of Defiance when it was released, in Path of Exile.
Partaking in a giant "MergerSmash" in Planetside2. 336vs336 of pure madness.