A few times in the last couple of weeks under heavy load, we've gotten some Mongo::Auth::Unauthorized errors. Restarting our app fixes this problem. We're on Mongo 3.0.11 and all our databases have the auth scheme upgraded to use SCRAM-SHA1 auth.

The stack trace we get shows the driver using cr, and looking at the mongoS, we can confirm that, e.g.:

> May 30 07:10:05 iadapp2-1vz26 mongos.15426[31160]: [conn3853753] Failed to authenticate REDACTED@DATABASE with mechanism MONGODB-CR: AuthenticationFailed MONGODB-CR credentials missing in the user document

I wonder if there is a way in which, perhaps during a fast election or something, the driver can lose track of what features the server has.