Before I start, I should say that my DM had the excuse of being young and relatively inexperienced compared to his current self (we have been gaming together since we were 13). He has the good grace to acknowledge the cheese he pulled

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Okay, back in our glorious high school days, our DM was running a campaign that featured a Super Race. I won't say the actual name (I still quiver with impotent rage when I hear it), so I will call them the Betterthanyou. The Betterthanyou lived up to their name; no matter what our characters were up to, be it exploring a dark and dangerous forest or battling hordes of evil undead, the Betterthanyou would show up, sometimes in great numbers, other times just a few, and make us look like chumps. If we were fighting a dragon, and it was winning, sooner or later a Betterthanyou would show up to heal us, beat the crap out of the dragon, and lecture us about being careful before vanishing back to Betterthanyou Land. It was irksome.
I am happy to say that other than that, the campaign was awesome, and our DM did, eventually, get the hint and downplay the Betterthanyou.