As a DM I play 5e, and I tend to play fast and loose, often introducing different mechanics not in the book for monsters and NPCs. My players were in the middle of what was essentially a war zone, and decided to send off one character’s adopted kid to escape while they went to fight. This child wasn’t a fighter, and their only skill (as I described it) was that they were “Very fast and good at dodging”. So, the players decided to give the kid some tools.
One was a halfling with a spare set of armor (good size for an elf kid), another had a poison handaxe, one had a spare sword, and the paladin (usually the resident goofy joke character) gave such an awesome and powerful good-luck you-can-do-it we-believe-in-you speech that I gave the kid a point of inspiration. I was actually getting amused by the fact that they were doing so much for this NPC child who I fully intended to be alright, but eventually the kid was ready to be sent off.
DM (me): So this kid is decked out in humorously deadly gear now, you gonna send them off?
Monk: Yeah. I uh, I kneel down—"Just find my horse Breka and ride off to that one town until we can find you again, okay?“
DM: Uh, Sunny nods and looks really confident with themselves. They’re just gonna turn to set out towards that horse, they know where it is.
Ranger (speaking for the first time in a long bit): I wanna cast a buff on Sunny.
DM: You… oh. Okay, what’s it do?
Ranger: It uh, it adds to their base Dexterity score, [about 7] points, and it lasts for awhile so they have a lot of time to get away.
DM: It… what? That’s a spell?!
Ranger: Yeah, it’s a spell.
Barbarian (sitting next to her and leaning over): Yep, it’s a thing, I’m looking at it right now.
DM: I… okay, you… you do that. Then the kid runs off to find the horse, and… oh my god. Guys, what have you done.
Monk: What? What happened?
DM: …So because Sunny is a child and a small target, I decided to give them a special mechanic. When they’re attacked, they can choose to either just let their AC handle it—which you just boosted with that armor—or they can try and dodge the attack. And instead of AC, they… they contest the roll with Dexterity. They already had a base Dex of 17.
Group: *pause for a long moment before they begin laughing*
DM: Guys, you just made this kid basically unkillable! You just—they’re a demigod, now, this little elf kid scampers off to fight Thor or something, and Thor can’t land a hit! You—the kid is immortal! This kid has become a diety!
Monk: Protect my child! Not even god could hurt them now! God child!!
Group: *begin chanting “God child”*
DM: I—oh my god their initiative mod is insane now too, I—I think this kid just ascends into the heavens. I can’t believe you’ve done this.