With a gtx 560 ti (gigabyte) all my gpu tasks were in error.
Turning around on internet, it seems that this overclocked cards are tested for games but not with cuda. The cause of the problem would be insufficient voltage.

This problem seems independant of any driver etc. Lunatics was reported to work with gtx 560 ti; perhaps for some others but not for me.

There is two solutions:
- Lower the frequency, from ~9000Mhz to ~800 Mhz
- And/Or set the voltage of the gtx higher, from ~1V to ~1.065V)

Lower the frequency is easy from the NVidia config panel, but it's not a great solution. Buying an overclocked card and downclocking it...

On the other hand modifying the controls for voltage are grayed with almost all settings applications including NVidia control panel. - bug or design ? duno
More, many people are afraid to touch so critical settings.

I have found a program to modify voltage: MSI Afterburner works with cards from other vendors (oct 2012) and is freely downloadable.
I have now correct gpu completed tasks with my gtx 560 ti, with lastest NVidia drivers, and I have even overclocked for tests to 950Mhz

Finally my advice to all gtx 560 ti owners : raise the voltage

(I apologize for my bad english)

errata: read 'from 900Mhz to 800Mhz' (and not 9000Mhz !)