The story came from the mind of a 12 year old. Not a logical thinking persona kid telling it for shock value.

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A really awful scene imo.

It would be IF none of the members of the crowd had any blueberry pie. But we don't know if they had or hadn't, so we can't say if it's a plot hole for sure. Either way, it's perfectly acceptable for a story made up on the spot, told by a 12 year old, to other 12 year olds would have plot holes. But questioning it is like Vern asking whether or not Lard Ass had to pay to get in the contest. The answer is: "it doesn't matter, that's NOT the point, enjoy the story instead."
This is pure EVIL:

Ok then. I will ask again. Is it a plot hole in the story within the story.

It's not a "plot hole" because the plot of the film is not in any way affected by the supposed goof. It's a made up story within the story.
This is pure EVIL:

Best plot hole ever. But yes, everybody eats the pie not just the contestants.

Because iT makes the story funnier.

No one puked blueberries, as Gordy said, they upped "used blueberries."

Yes, but that wasn't the question.

The question wasn't really about the puking - it was "why would the audience puke blueberries since only the contestants ate them?".
I concur with the earlier poster who said that for moviemaking purposes, it was easier to mix up one big batch of stage puke than different types. I've seen the movie maybe 5-6 times since it came out and its something I had never noticed. The moviemakers probably figured we'd never notice as well!

I've always just assumed that the audience got sick by seeing the fat kid puke so much.

If this wasn't a kid's silly story, I'd be wondering about this too. Alright, maybe I have.
You want something corny? You got it!

Not a plot hole of either the movie or the story Gordie told.
Reality wise, making a big batch of fake blueberry pie puke was easier and less expensive than having different colored/consistency puke for the other characters in the scene. Within the context of the movie, it's reasonable (in my opinion) that the other boys would be picturing everyone puking blueberry pie since that's how the puking started.
Plot holes effect the plot of the movie. You're talking about a "goof" or inconsistency.

Vern?
Ephemeron.

Lmao. Yes, someone else noticed this too!

It was a fake story, it wasn't supposed to actually have happened in the movie.

It's a story made up by a 12 year old.

It was kinda a bake-off, so maybe the audience members HAD eaten pie as well.