Silent failure when inserting mongo object with an object field as the _id

I accidentally inserted documents in the database with an object as the _id field. I noticed this person reported a similar problem a few years ago: http://stackoverflow.com/questions/24618380/mongodb-inserts-silently-fail

If I try to do a find all query {}, I can't find the object inserted, but no error was returned. I'm on Mongo 3.0.

When an index sees a non-object-id object in the indexed field, does it just skip over it rather than erroring? Even if this was the case, I'd expect to find the object when searching with an empty query.