One of my favorite moments as a DM was... wait, I need some backstory for this.
The initial campaign relevant to this story, was a long running campaign where the players had become pretty... unique. One of the players (this was actually his first time playing a tabletop rpg) had been turned into a werebear. He had a cockroach mount (ankheg), that I tried to have an NPC buy off him for an obscene amount of gold. He refused. He genuinely cared for it. He also had a Cerebral Hood (from the 3.5 Fiend Folio) symbiot as a friend. To give you an idea, it's like a manta ray that wraps itself around your face, and sticks its tail down your throat (they called it his "face-f*cker", lovingly, of course). He had some crazy good rollplaying to really bond with it, and they genuinely made a fun and interesting pair.
The groups quest was to do something about a meteorite that was corrupting the area in a giant cave underneath a hollow mountain. They weren't very good at planning, or finding out information, or basically anything you should probably do to make sure things go well. It culminated with them standing at the meteorite, with an alien army surrounding and attacking them. The wizard of the group tried to use his knowledge of the arcane to give him an idea of what to do, and the best he could get was a ritual that involved some ley-lines and a fireball cast at the meteorite. Everyone recognized the awfulness of this plan, and decided it was the one they should go with.
I told them all the resulting explosion killed everything within about 100 miles of where they were, and ended the campaign (I'm a jerk, I know).
A couple unrelated campaigns later, we are starting a game in a new campaign, and unbeknownst to the players, it's the original campaign, but about 10 years in the future. They played through months of this new game without having any idea that it was the same world.
Then, as they were rowing across an underground lake, in a cavern deep near a mountain, they saw something moving above the water.
As it got closer, they saw it was a Cerebral Hood.
The "Aw man!" "No way!" and "Holy crap!"'s at this point were pretty exciting, but as I had the face-f*cker communicate with them, telling them that he was sent there by his father, who was the trusted consort of the Bear-King of Undermountain, the player of the werebear from the past campaign (who was now playing a were-wyvern, or werevern as we called him) was very nearly in tears. The crowning moment, was when this thing proceeded to tell him that the Bear-King was his father, and that although his destiny lied elsewhere at the moment, they would eventually be reunited.