This isn't an in-game story, but the result was pretty crazy, and is still one of my favorite character designs.
A number of years back, my brother was playing in a Rifts: Phase World campaign. The GM was really into this major NPC of the setting, Thraxis, who is basically a demigod who is also the richest character in the setting, and had imported a D&D character of his own and her patron goddess, and effectively the campaign was all about the NPCs, with the PCs being relatively pointless. So my brother, having had his previous character die during an invasion of nasty opponents that the NPCs then killed, came to me with a request: "Help me make the most insane and twinkish character you can come up with via the rules."
Well, I couldn't say no to that. I remembered a few interesting rules snippets I'd seen when reading various sourcebooks, and had been intending to make this character anyways, so I gladly donated him to the cause.
You see, Rifts is a game involving transdimensional displacement, and the setting actively encourages bringing characters from other games over - especially other games by the same publisher. There was even a conversion book created specifically for this purpose! And one of the other games they published was Teenage Mutant Ninja Turtles. Now, TMNT had a number of sourcebooks for it as well, and the rules weren't quite the same for the two, and we exploited that for all it was worth...
One of the major rules that had gotten me thinking was that mutant dinosaurs from one of the supplements involving time travel became Mega-Damage Creatures with supernatural strength upon transfer to the Rifts setting. This made sense, since regular dinosaurs in the setting were already Mega-Damage creatures. However, the rule was effectively "convert SDC (TMNT and Rifts hit points) to MDC (Rifts Uber-hit points used for mecha and dragons) at a 1-to-1 ratio." This allowed us to twink our character in interesting ways, because the bonus SDC from physical skills was normally not applied to MDC characters, but would be here (as MDC!) due to the conversion process.
So, we chose a dinosaur. A Triceratops to be specific. My brother decided he would be named Booldemug. Now, in TMNT, the creation process uses Mutation points to allow you to determine exactly how your character would be mutated from a base template. Our Triceratops had no mutation points, but it turns out that Triceratops characters are somewhat imbalanced because their size is great enough that when you trade in size levels you end up with more Mutation points than most characters did at an equivalent size level. But instead of shrinking Booldemug to ogre or human size and spending half those points on giving the guy bipedal traits, hands, and speech like normal sane people did, we spent all of his size levels to gain mutation points, and didn't spend any on humanizing him at all. Instead we spent the points on all the considerable armor a triceratops could get, (with the remarkable SDC bonuses that conferred) a full set of horns that did more damage than any melee weapon in the game, and the Time Magician background from the same supplement. With the loss of size, our dinosaur had greatly reduced strength, but he had enough skill points to buy every physical skill in the game, shooting back past his original strength, gaining even more SDC, and large bonuses to all his other physical stats. Also, his speed increased a lot by being so small. And in TMNT, unlike Rifts, high speed scores increased your dodge bonus, which then got transferred wholesale to the new setting.
By this point, we had a cockroach-sized triceratops who could time travel, punch through steel plate, had hit points in the thousands at first level, and could only be hit on a natural 20.
But things got more insane. We deliberately ignored the animal psionics as they were mutually exclusive with the more powerful Rifts psionics. We instead took a gamble at rolling on the awakened psionic powers in the conversion book, and scored big time. Major psionics, allowing Booldemug to pick up Telepathy and Telekinesis! Booldemug's communication problems were over, and he was now capable of self-propulsion using his psionics to move around at his ridiculous speed score. He also managed to pick up super powers! We went with Invulnerability, supplementing his ludicrous MDC capacity even further and giving him innate life support. Also, as a dinosaur his strength became supernatural during the conversion, allowing him to deal Mega Damage. Furthermore, he was now allowed to pick an OCC! (Character class, using non-Rifts parlance.) We went with Priest, granting Booldemug some quite potent spellcasting abilities as well as a bunch of interesting esoteric abilities. Finally, we picked his patron deity: The DM's old character. He'd granted her divinity at some point, so we felt it was perfectly natural she'd have some fervently annoyingly devout worshippers, and Booldemug was one of them.
So, in the end, we had a level one cockroach-sized super-speed psychic triceratops who could time travel, breathe in space, fly around like a bullet, cast spells, heal people, punch through starship hulls, had more MDC than most ancient dragons, giant mechs, or small starships, could still effectively only be hit on a natural 20, and could get away with annoying the GM's pet character because any retaliation against her ardent worshiper would have made her look like an utter tool. We stuck him in a personalized mech suit and let him loose to much hilarity.