While working on SERVER-103281 I found that DocumentSourceGeoNear::validatePipelinePosition() complained for some reason. Looking at the logs I think the pipeline that was being validated didn't have the $geoNear in it, so it might have to do with the splitting (this only happened in sharded scenarios).

Also noteworthy, such a command on a sharded collection fails even though the explain works, while on a sharded clusted with an unsharded collection the command works but the explain fails.