I have a tradition of making the first encounter of a game starting at level 1 a gang of housecats. They have claimed a few characters over the years. It serves both as a way to break the ice for new players, but also as a humble reminder that sometimes, you get fucked for no good reason.
Another time, a random NPC (a baker, if I remember correctly) one of my players murdered early on kept coming back, increasingly reanimated into stronger and stronger bodies. He wound up replacing the BBEG I had planned on.
By the end of the campaign, he had become an undead cyborg vampire, with touches of fey, demonic, devilish, and angelic origin, had become a petty god of sea birds, had become an efreet, and then became a psionic and literally willed himself back to life one last time, whatever that makes him. He straight-up murdered the previous BBEG after his final big speech in the last fight, and went down for good that time.
After the first fight, he got his own theme, and I've sometimes screwed with those players since by either playing something similar, or just "accidentally" revealing it on the playlist for the night.
He did come back one more time in a game that specifically had a tournament full of weirdos, but that doesn't really count.