In actual operation, nothing was. Both Viking Mars shots had specifications for the length of time they were to remain in operation. While I do not recall the time span, both exceeded the specification by years. I do recall that JPL had to scrounge additional funds to keep the data coming in from all the deep-space probes, including the Vikings, as the deep space mechanisms were all working for far longer than expected. Surely any engineered artifact which lasts for longer than its design specification must be considered a success.
Nothing lasts forever, especially that most fragile of all artifacts, software.
Thus the fact that the Viking 1 Lander software was scrambled beyond recovery some 8 years after the Mars landing only reminds one that the software is one of the components of an artifact likely to fail. So I see nothing remarkable about this event, nor does it in any way detract from judging both Viking Mars missions as unqualified engineering successes. I have been intrigued by work that has been going on at CMU to support the ITC Distributed File System. The authentication and secure transmission functions are provided as part of a connection-based communication package, based on the remote procedure call paradigm. At connection establishment time, Vice and Virture are viewed as mutually suspicious parties sharing a common encryption key.