An aggregation that names no sort found more than its limit groups. Its first limit groups in key order would be a cut nobody asked for: a top-N needs a sort, and every group needs a higher limit.
A stable, machine-readable code; a contract: codes are added, never renamed.
What the binding error names: the absolute logical field (or "") for a model rule, a request part otherwise.
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Its own problem, not the generic invalid query: the query is well-formed, its answer too large.