There is plenty to question the guy upstairs about. Or gal. Heck, maybe even bunny rabbit.
As far as the rock goes, if it were possible, then the god in question wouldn't be all powerful. If it were possible, then it still negates the all powerful part since its something god could not do, but in a less negative sense.
ts essentially why questions that lead to that kind of reasoning are frowned upon among "dedicated" followers of organized religion. Blows the whole thing out of the water at that point.