this error handling pattern isn't obvious at first

error_handling.go

func f() error {
    // might return an error
}

func somethingElse() {
    switch err := f(); err {
    case mgo.ErrNotFound:
        // a specific error that is recoverable
    case nil:
        // no error occurred.  Usually this is just a break statement.
    default:
        // all other errors get handled here.  This is where you want err to be named, so that you can reference it to either return it or write it to a log file.
    }
}