Just came back from visiting friends of Mrs GF, and we had a great time. On the way out, our host mentioned something about taking a "quick" look at his radiator. He has a 2015 Malibu with slightly over 100K on the clock and he wanted to change the coolant. Apparently the coolant had become "sticky" -- I looked at the radiator and indeed the stuff was like mud in the upper radiator.
My first inclination was to ask if he's mixed a glycol-based coolant with Dex, and he answered "no" to that question. I've seen Dex-Cool systems develop sludge after more than five years, but never like this. We quickly flushed the radiator and sure enough -- the sludge poured out the bottom of the radiator hose. The radiator seemed to clean up nicely, but at that point the entire system was likely in the same condition. I told him to flush the entire cooling system thoroughly and refill with Dexron. It only took a few minutes of my time, but during the flushing procedure, his wife mentioned that she had been on a trip last year and stopped to get an oil change at a local gas station -- they topped off all the fluids (incl. filling the expansion tank), and I suspect that they mistakenly added regular green glycol-based coolant to the system; which in turn caused this mess a year later.
Be careful -- watch what people add to your systems when you take it in for service. Better yet, educate your wives, girlfriends, family members on making sure the proper fluids go into your modern cars.