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I'll say that calling these things a Plot Hole is part of why Plot Hole is now definition-only; these things aren't logical issues that render the plot impossible as presented, they're just Fridge Logic-y questions.
There may be a trope about extras being just scene filler, but that's also the point of extras. The Main Characters Do Everything is worth looking at.
Currently Working On: Incorruptible Pure Pureness
War Jay, please clarify what you mean by Plot Hole is "definition only"?
...See Definition-Only. That's what Plot Hole is. It cannot have examples, though it can still have wicks. I took it to TRS because of severe misuse.
Currently Working On: Incorruptible Pure Pureness
In the TV show "Herman's Head" there are other people who work in the office, but they are never shown interacting with the cast. It's to the point where it doesn't even make sense to have them there. Why would the boss call staff meetings and never invite the "other people in the office."
Cheers might also qualify since the bargoers outside of Norm, Cliff, and Frasier rarely are remembered or frequently noted by the bartenders.
Additionally, there have always been questions in Glee about the backup band. If there guys are a small band of outcasts, is the band lesser outcasts? How come they aren't even part of the social clique?