I have a GTX 560 and it is overclocked to 950 mhz.
I was running MSI Kombuster and after about 20 seconds in I just hear my fan cut out.
I have ran this test many many times with the exact same settings and clock speeds and everything and had no problems.
So now my fan is stuck on the lowest speed. I had to reset to stock clocks and I still idle at like 53 celsius.
I have tried all sorts of things to get the fan to cut back up to full power with no luck.
The fan does run though.
Edit: it came back on to high just randomly by itself. then went back to low a little later on.