Way back when I was like 8-9 years old my brother played a prank on me, he created a character named ABD Inc. and sent my player a letter saying that my file would be deleted if I didn't pay like 10 mil bells in 24 hours, scared tf out of me and I still hate him for it lmao :,,,,)
Oh man, that's a tough one! There have been so many it's hard to choose just one! Catching an incredibly rare fish, finishing a collection of some sort, paying off the final debt, etc.
Since I don't have a switch yet, and have a urge to play animal crossing on the switch, I would say it would be between Pokemon, and ACNL so far at the moment.