Just so it's not all doom and gloom in this thread, sometimes things can work out.
I wanted to go to Ottawa for a couple years (For the GF's schooling), the CM stated "Oh but we were planning on keeping you here for a year…" and then a couple weeks later, I got my posting message, for Ottawa. Fast forward two years, and I'm getting posted back to the west coast, just as I would have liked.
It really just depends upon what holes the CM has to fill with what pegs. And I suppose whether or not the move budget has been expended or not.