The update subsystem enforces constraints on the document structure using different C++ code than the insert command. This has led to now-fixed bugs where a document could have had

• multiple top-level _id fields (SERVER-75879) or
• _id field as an embedded object containing a $-prefixed field name at any level (SERVER-75517, SERVER-75880).

Additional validation has been introduced into the update subsystem in an upcoming MongoDB version to prevent further introduction of these malformed documents. It would be useful to provide customers a means to identify and remediate any existing documents. This is because future versions of the server may not react in a well-defined way to these malformed documents. Adding checks on the _id field to both the validate and dbCheck commands would make it possible for customers to sooner learn if they are impacted and address the undesirable behavior in their application.