I'm running 1.4 mongod with --notablescan, I keep seeing this in my logs

runQuery: production.documents{ query:

{ _id: "ef1f5cd0e378bfff528ce7", accessor_id: 26 }

, orderby:

{ created_at: 1 }

}
running multiple plans
User Exception table scans not allowed: production.documents

I'm not sure if this can be related to the database sometimes locking without any other explanation.