I hope somebody reads down this far...
The stupidest thing I've ever seen an author do to their story was what David Eddings did to his story in Book 4 of the Dreamers.
He revealed that the "Eldar Gods" hadn't been living forever, but had only just been created immediately before the first book began. This completely slaughters big chunks of the plot. Plot point: the "Elder Gods" tired after thousands of years of being concious? Nope, just made that way. Zelana a ***** because of her own life choices? Nope, just made that way. Argh!
Worse yet, foreign armies had been brought in to fight the evil bug bad guys in the first book because the Gods needed armies. Then it's revealed that the Gods can handle things after all, and the armies are just there to bear witness not to mess with said Gods. Then it's revealed that there are actually super-gods that could settle this whole nonsense in a couple minutes without any hassle. But they don't, not until the end, and not for any good reason. The villains, on the other hand, start out terrifying, but get no such upgrades and by the end are just a bunch of pathetic insects, doomed and helpless.
And to put the capstone of stupidity on the whole thing, at the end the super-gods turn back time, erasing the entire plot from existence, along with the baddies. Why couldn't they have just done this to begin with instead of making gods and playing riddle games and sneaking around and being witty? It would have saved us four books of increasing nonsense and pointlessness.