WoW. I sank hundreds of hours in vanilla. Spent forever getting tier 3 armor completed and most end game content completed and then Burning Crusade was launched. I was so excited that myself and about 15 guildmates road tripped from across the country to meet up and play in person for launch.
I remember crossing the portal and within 15 minutes a common mob dropped an item that was better than one of my tier 3 armor pieces. Pieces I had spent weeks farming for. I was so frustrated that I couldn't enjoy the next 2 days playing, even with everyone there. I got home, played a few more days, and had already replaced half my armor. I quit WoW cold turkey and since then have only logged in once in all these years.
I wasn't even that mad that my armor was replaced, I was mad that it was replaced so quickly, and not even at boss fights. It was normal mobs dropping rares.