SERVER-131924 began recording query stats entries for reads that error. This introduced a new class of stored shapes: shapes from queries that can never succeed on the node that recorded them. One that was exposed: $$CLUSTER_TIME on a standalone. 

For an aggregate, the following flow is taken: command referencing $$CLUSTER_TIME while being on a standalone parses successes -> runs registerRequest() so key is live -> throws 10071200 in initializeReferencedSystemVariables(). Because the key is live, the errored shape is recorded. When $queryStats is called later, toDocument throws the same code when it hits initializeReferencedSystemVariables() on this path:

toDocument-> computeQueryStatsKey
    -> AggCmdShape::appendCmdSpecificShapeComponents
    -> pipeline_factory::makePipeline
    -> finalizePipeline -> initializeReferencedSystemVariables()

Current workaround: This error code was added to the exclusion list in query_stats_stage.cpp:235.

Github thread: https://github.com/10gen/mongo/pull/59526#discussion_r3684942180