I guess I should clarify a bit more Neuro. I would have expected your explaination if all the test shots at f/1.8 were a little OOF than the f/1.2 shots. In all but this one instance, the f/1.2 shot was sharper than the f/1.8 shot. I also have to confess, I was looking at these images at 200% and I'm probably a closet pixel peeper.
Apologies, but I don't understand your clarification. Are you saying that in general, f/1.2 was sharper than f/1.8, or in one specific shot, f/1.2 was sharper, but the rest of the time, f/1.8 was sharper?
What I'm saying is that with correct focus, f/1.8 will always be sharper than f/1.2, that's just basic optics - you stop down from wide open and the lens gets sharper. MTF tests and/or ISO 12233 test shots, which when properly done are perfectly focused (either with 10x Live View, or better yet by focus bracketing and picking the best shot post hoc). But, what I'm also saying is that if you are using AF for your shots, f/1.2 will usually be sharper than f/1.8, because the f/1.2 shots will be correctly focused, while f/1.8 will be back focused because of the focus shift (focus shift does not apply wide open). However, that also assumes the lens is properly calibrated on your body - for example, if it actually front-focuses at f/1.2, then the focus shift at f/1.8 may result in correct focus (two wrongs making a right, so to speak).