I used to have a VERY strange issue come up on my C55. About once a year, the A/C would turn off completely and not do anything (no air coming out of the vents). You'd notice it instantly because it'd suddenly get very hot inside the car. Not good when you live in a warm climate...
It would only last about 30-60 seconds, normally at startup, then the HVAC would kick on. I was never able to resolve what the issue was, but it happened so rarely that I just lived with it. I figured it was something electronic in the SAM.