Priority: Major - P3
See also related
SERVER-12209 ticket where this issue came up.
Sending a document with _id of extended JSON form to a primary will succeed but crash all secondaries immediately upon replication.
Some drivers seem to validate for such cases (key starts with $ in nested sub-document) and reject the write, while others (I only tested with Python so far) don't. The behavior should be consistent.
Another discussion point is whether these validations should happen on the driver or server side. Currently --objcheck seems not to protect the server from such _ids.