AggregationRequest validation unit tests, like https://github.com/mongodb/mongo/blob/25818a31c7d8209aa8ae22a536f94725aa7fa21e/src/mongo/db/pipeline/aggregation_request_test.cpp#L466, do not precisely distinguish the outputs and, therefore, run a risk of not catching AggregationRequest validation bugs. The same error code is used to report multiple error conditions, and more precise validation error cause can only be determined by examining the error message.