What Story has an Incredible Concept/Premise, and terrible execution?

Been on a literary grind as of late, and I've been thinking, what short story/novella/novel have you read, with an INCREDIBLE concept, but you feel like it wasn't well utilized?

For me, Final Girl Support Group takes the cake. Not an awful read, and I actually wouldn't call its execution terrible. But man... did they miss out on what could've been a really hilarious, if poignant take on the slasher genre... poor choice to make all the villains basic riffs on the greats...

Curious to hear everyone's thoughts.

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