I am unsure if this is a bug or what, so I am throwing it on this forum (NT) and I'll be bug reporting it to.

When calming 'normal' mobs I get what i used to belive was the boss message.

This mob cannot be affected by calms.

I will try again and it may counter, or blackhole, or fumble, but if none of those happens it may calm, or I might get the message again.

If you keep trying it will calm.

Odd?