A user can input a query such as 

db.coll.find({ loc: { $near:  {$geometry: 1 ,y: 327391, z: 1} } })

where this is treated as a legacy style query, where the the point is taken to be 1, 327391 and 1 is the distance. See GeoParser::parsePointWithMaxDistance(). 

However, the serialization function for geo doesn't consider this to be a valid option and only considers legacy coordinate pairs, and not triplets. This may only affect query stats and debugging since we only use the serialization function when the policy is not LiteralSerializationPolicy::kUnchanged. Query stats errors since the serialization function will check for [x,y] pair, and then when its not found, will go through $geometry serialization. But [x,y,z] all be numbers is also valid.