https://github.com/mongodb/mongo/commit/7c8b3c68da9dd93641c59e6cff7a47a264fd68b4#diff-87ac423ce6fa7bab76c529938c1fa2b4R168 added this error message:
'It is not possible to authenticate as the __system user on servers started without --keyFile parameter'
It should probably say 'without the --keyFile parameter' or 'without a --keyFile parameter' instead.
- is related to
SERVER-16009 Uninitialized value used in SCRAM code