1. Create a collection that has an item with a float set to NaN
2. Export the collection with the "JSON - mongo shell / 3T MongoChef" option selected.
3. Try to import the json file back in using MongoChef. It will fail on a parsing error as the NaN value was encoded as 0xEFBFBD

Exporting with "JSON - mongoexport" works, it is just the mongo shell one that fails.