Recently, BF-31428 occurred because a query with an invalid $hint was executed. $hint is supposed to be a string or an object, but in this case it was a double.

This caused the above issue with query stats since it reasonably does not expect that syntactically invalid statements will be executed. I think the fix there is to make query stats more robust to this, but the behavior of $hint remains confusing.

This is easy to reproduce from the Mongo shell:

db.nonexistant.find().hint({$hint: -1})

This command succeeds even though $hint has the wrong type.