What is "Fool's Crumb" and how can you spot it?

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Been doing some reading up (seems the reading up on creating sourdough bread never ends...) and ran across Trevor Wilson's term "Fool's Crumb." I saw his instagram photo of such, but now I'm just confused. He didn't really explain it well enough. So i searched around here and found a lot of threads asking "is this Fool's crumb" with photos...but no thread where Fool's crumb is really explained.

Obviously, it looks like the real deal. Very much like the crumb in a well proofed and fully baked sourdough loaf...but somehow, it's not the real deal.

Why not? And why isn't it, well "good enough" compared to the real thing? My novice self certainly can't tell the difference from any of the photos, and if I got "Fool's Crumb" I'd probably be super happy because, hey, if it's not over/under proofed (hallelujah!) I'll take it and send up prayers of thanks to the sourdough gods.

Can someone post pictures of "Fool's Crumb" and explain what the tell-tale signs of it are, exactly?  How does it happen? And what makes for real crumb vs. fool's? Trevor compares it to Fool's Gold, but is that really fair? I can't get any money for Fool's Gold as I would for real gold...but I imagine I can enjoy eating the Fool's Crumb almost as much as eating bread with real crumb....

My understanding is it's a few huge holes in the top of the loaf that are really a sign of underproofing versus a nice open crumb with lots of well dispersed holes that vary in size. Think of a loaf with 3 huge gaping holes versus dozens of differently sized holes. 

Thanks, that helps. and the pictures really help. I can tell that the crumb in that picture isn't right even if it's not super obvious (like ragged holes).