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I had two life threatening episodes during Labor Day weekend a year apart.  Couldn't shut down cruise even when jamming on brakes.  Went through stop at 65 mph in area where cycle group had been cruising.  Last Labor Day came up behind school bus and had to hit plastic cone separating lanes to avoid the bus.  GM can't verify problem, say they don't know what's causing it, and can't do anything about it. 

First of all, if this ever happens again, just turn the key off and the engine will stop allowing for adequate braking. Secondly, I know it's a stressful situation when you can't stop your car, but did you try hitting cancel on the cruise control in addition to depressing the brake pedal? Something definitely isn't getting the message to disengage the cruise control and I'm sure with the occurances happening so far apart, it's a very hard problem to duplicate. Still, I would ask the dealership's service manager to consider replacing the cruise control actuator and/or the PCM.