When a default value for a document is different from the value in the database, the content history detects this as a change. But Doctrine doesn't write the change to the database.
This means it keeps generating records in the content history.
Example 2: reference has wrong db (it would be better to turn storage of $db off, this will be the default in Doctrine ODM 2, http://docs.doctrine-project.org/projects/doctrine-mongodb-odm/en/latest/reference/reference-mapping.html)
Note: also when the changeset is empty a record is sometimes written to the content history.