What was changed into an immortal object? In the first arc of Sword Art Online (episode 14, ~9:40), the boss whom Kirito fights activates something in a system console, and then a notice is displayed: "Chaged[sic] into Immortal Object".



However, this message does not seem to make sense. Despite Kirito's request,


   Asuna is slain by a one-hit KO as she attempts to defend him, and Kirito is consequently killed as well. His remaining consciousness then gives one more blow to Heathcliff, who dies. Thus, it is clear that none of these three characters was given immortal status.


Was this an error, in that the message was supposed to refer to this boss removing his own immortality? If not, what exactly did this message signify?
As you say yourself:


  Was this an error, in that the message was supposed to refer to this boss removing his own immortality? If not, what exactly did this message signify?


While googling, I came across some posts on a forum which seems to say that it is a wrongly translated message and that it should say something like: "immortality status changed".
I'm not sure if it's accurate but it seems to tell the exact same thing.