@[HWU]Melkor:"What should I have to paste here?"
Generally, only post full details of an error if nobody has already described the same sort of error. In that case, describe what went wrong and you might copy and paste the abnormal parts of the Results Status log.
If your type of error has already been reported, just state the type of error, eg "hung at 99.415%".

Reading armstrdj's post again, in this thread we were asked to paste in the last few lines of the stderr.txt file in the error WU's Slot directory. Sorry I mislead you by suggesting you look at the Results Status log. The slot directory is cleared when the WU ends, so your stderr.txt is lost. Only the log in your WCG Results Status remains.

You report having the default 50% throttle setting so I think that your error happened because BOINC currently can't re-start GPU WUs after they have paused during throttling. Your BOINC version etc is useful info because by examining a number of these situations the techs may discover another factor that needs to be present to trigger the error.
You have already given enough details I think

Looks like there needs to be a separate WU-cache for GPU-based WUs so that the logic for calculations of cache-sizes are independent of one another. That is: the CPU-based-WUs interact only with the cache for CPU-based-WUs while GPU-based-WUs interact only with the cache for GPU-based-WUs.
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Though there is as yet no separate buffer control for CPU and GPU [no one has made a compelling case at the developers FAICR for such user configurable settings], GPU and CPU have separate schedulers. Knreed posted that he'd fixed some bug, working with Berkeley to stop this occurring, but seemingly... At any rate, my latest 7 test client still has a single DCF per project [speak, WCG is a single project to BOINC], and the <dont_use_dcf/> entry is there but not operational. Choice *was* a number of months ago not to use that feature [disabling the DCF in effect]. Maybe/maybe not they are related. Long as the run time estimates are set right for GPU Beta, this should not affect CPU side, moreover it's still Beta, so there is no cross feeding of run times averages between GPU and CPU, don't expect there to be since is is a separate feed. This is something which has all to be learned on the go with this first for WCG.

SekeRob, you mentioned in your other posts, that the DCF is now on the server. Now is that a DCF for a CPU-based-WU? We may need to have a DCF for a GPU-based-WU to operate on a cache for GPU-based-WUs. I can imagine further difficulties if GPUs are not given its own space so to speak, and force GPUs to be 'just another CPU' in a manner of speaking. Already the over-downloading of CPU-based WUs is apparently caused by the servers misreading a reported done-GPU-based-WU as reflective of a client's CPU-performance and may have thus triggered a server command to fill the client's cache based on a DCF that is supposedly for CPU-based-WUs only but which DCF the GPU-based-WUs performance may have modified.
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Looks like this wave of beta tasks is finished, just my two cents: I got 4 beta tasks on one of my machines and first of them was stuck on 99.415%. The machine is a notebook winth Windows7 54bit and nVida 540M GPU and to avoid overheating, throttle is on - use max 50% of cpu time.
I turned throttle off, restarted BOINC and all beta tasks finished successfully. I observed that during computation, both CPU core and GPU were used in 100%, despite of throttle settings.
I hope that this issue will be solved quickly - I think that a lot of people use this setting because of heat and noise, using 100% of a notebook CPU is not an option for me.
EDIT: just to avoid confusion, "issue" means hanged workunits, not 100% CPU utilisation :)