The mongodump tool stops when it encounters a collection for which it doesn't have permissions.

If a database has a large number of collections, the administrator may erroneously assume that the dump was successful based on the fact that "most" of the collections were backed up, especially if the dump is run in a cron context.

Mongodump should backup every collection that it has permissions for. (Failing completely without backing anything up would also be preferable to backing up a random subset of collections)