For me, it would've been two things:
1. The Climax of Sonic 3 & Knuckles. God dang it, it so good (this will be explained shortly). After beating the heck out of Eggman in Lava Reef, you finally get to 1v1 Knuckles.
Let's just say that i tried goofing around with the AI of Knuckles, so please don't kill me for that . Once you ALSO beat the heck out of him, he finally realises the collosal mistake he has done after seeing Eggman taking his shiny Master Emerald. We get to Sky Sanctuary, and as soon as we got near the entrance to the Death Egg, a over-buffed robotical version of Sonic decides to beat the heck out of you (karma moment). You eventually do the UNO reverse and destroy him, and you get to the Death Egg. I'm pretty sure you know how the rest of the story goes.
2. Metal Sonic Race in Sonic CD. When i first saw this metal punk, i was thinking: "Ooh, look at that robot, he is gonna assist me now!" (I was playing on my brother's save file in Gems Collection, so i didn't knew you will see him in Collision Chaos), but then Eggman decided to actually be half-smart and make an insta-death laser that he would not be used until 30 years later. Once i saw that, there was no messing around, as we had to show who is the fastest boi on the Little Planet. I failed many, many times, but it was worth it as we actually saved Amy after the race (who i had a bit of a crush on when i was younger, please dont kill me for that aswell
)!