The black screen is a very rare occurence, but the fan pwn polarity reversing tends to occur without much playing with,
I once forgot i had gpu-z running and my GPU Fan' reversed pwm polarity while playing HL2 and the fan jumped up to 100% because the temp hit 100c, as the fan was running at 0 RPM.
I was running Orbu2k's GPU Observer earlier and my fan reversed, last thing i saw before black screen was 100% @ 100RPM on LCD display and i got a black screen while doing some light gpu usage in a Flash video. I'm thinking the multiple sensors are causing the driver to crash without recovery.
from what i've observed this issue is only likely to occur if both apps can monitor the gpu fan, since i used to run speedfan alongside Everest just fine for fan speed controlling, and speedfan can display GPU temps (in builds of speedfan that don't display gpu fan speed, 4.41)
Other users have experienced this with Precision/Rivatuner vs Hardware Monitor/Everest (and now AIDA64)
My configuration is a i7 920, Asus Rampage II Gene, EVGA GTX 275, Auzen Prelude 7.1, with 6GB of ram.
that said, its perfectly fine when Aida or the other apps are run by themselves, but since people tend to like setting up fan configurations in afterburner and monitoring fps with precision/afterburner, etc its a bit of a problem when the gpu fan glitches out.
Basically what im asking is if the access to the fan controller can be made a little bit more safer.... actually i was messing with nvidia system tools device rule awhile back to set up a custom fan scale and that would glitch up as well while everest was running (haven't tried in aida64)
From what i've gathered the problem can occur when run along side
GPU-Z
Nvidia Inspector (or Orbu2k's GPU Observer Gadget which afaik just connects to the Ntune component in system tools)
Afterburner, Precision or Rivatuner
HW Monitor
Nvidia System Tools