Generally I always feel a tad bit guilty whenever I decieve someone, because I know how bad it feels <and I get decieved alot :D>. But I always try to banter my way out of it, and I *always* offer the person I am stabbing something, like a janissary position or something. (Never too late to manipulate, as I always say).
My worst stab has to be once where it was a game long ally that I had had a really good relationship with. I decided to give them a long explanation as to why I felt I had to do it, and how I felt they played a great game. I wanted them to see this message at the same time as they saw the stab.
I was in a huge rush though, so I sent this message *before* the moves went through. Three minutes before, to be exact. It didn't cross my mind that this may have been foolish as I rushed out of my room to catch a bus.
The next day, I log on, expecting a four centre gain and a heartbroken ally. Instead I find that they had logged on AFTER ME BUT BEFORE THE PHASE WENT THROUGH. And so, seeing my message, sending a courteous "wtf" they speedily changed their orders and stopped my betrayal in its tracks.
It was a rather disheartening experience for me on many levels.