hang on now
lets listen to what you are all saying....
ok now lets just get this straight. If Grievious was indeed the prototype/test case for Vader then Sidious knew all long that Obi would hack Vader up and leave his body a crispy mess on the banks of the lava river.
what complete rubbish
1- Palpy already owned Vaders soul, sending him to get crispy fried would make no sense
2- In the novel or the audio book or something doesn't Palpy 'run' to his ship when he senses Vader is in trouble?
There are bound to be more points than this but two is a start. There is no way Sidious knew what would happen to Anakin so there would be no reason for him to 'need' a prototype