Inserting into `system.indexes` is disallowed, but an explicitly create collection command goes through. This has some interesting, problematic side-effects. For example:

1. Creating a real collection on the KVStorageEngine.
2. Replicating with a create collection oplog entry.
3. Dropping a database skips dropping the collection.
4. Others? system.indexes is often treated specially in the code.

Particularly between 2 and 3, this prevents us from properly verifying a legally ordered sequence of updates on all document in WiredTiger.

I would imagine this is also possible on earlier versions, but I haven't verified.