Cmon people I expected more plausible hits to my previous post... Check this out...

Kirkland and Smith meet and there is a black coffee remaining.http://www.youtube.com/watch?v=sKVj-_TAHeQ



Haffner drinks black coffee.
http://www.youtube.com/watch?v=9z20aYkfg1s



That's why I believe Kirkland is Haffner's client... Haffner works for Kirkland.That's why Haffner always asks about Red John and always say:
"I don't care, I got clients that care".
Yeah clients.... you mean Robert Kirkland! :P

Hope I convinced you this time its a very well hidden clue and it popped to my head by luck!
Trust the subconscious mind!! :D