RFC 959, "File Transfer Protocol", October 1985Source of RFC: Legacy
Area Assignment: app
Errata ID: 2024
Reported By: John Klensin
Date Reported: 2010-01-27
Verifier Name: Alexey Melnikov
Date Verified: 2010-02-10
Section 4.1.3 says:
In the description of the SYST command, the second sentence reads: "The reply shall have as its first word one of the system names listed in the current version of the Assigned Numbers document ." Where  points to the then-current RFC 943.
It should say:
The reply shall have as its first word one of the system names listed in the current version of the IANA "Operating System Names" Registry (<http://www.iana.org/assignments/operating-system-names> at the time of this writing)
RFC 943 was several times obsolete by the time the community discontinued regular updates to the "Assigned Numbers" RFCs (see RFC 3232, January 2002). The clear intent was that SYST be able to use operating system names from that registry. An erratum pointing to the registry itself may aid the confused as well as providing better tracking and serving as placeholder in case RFC 959 is ever updated.