Camera not focusing - lens or body?

I have a newish to me 5D2 (with a return policy) and a 24-105 lens. I have had the lens for a while, no issues previously. On two occasions, I have been shooting and have gone to focus and shoot, and on pressing the focus button (AF-ON for my setup) it seems like the auto-aperture hangs, the camera confirms focus even though it's not focusing. By auto-aperture hangs, I mean that it seems like the aperture is stopped down when it should not be (the view through the viewfinder becomes darker, like using a MF lens). So I think either the lens blades are sticking or the camera is giving it an incorrect signal.

At this point, I am not sure if it's a camera issue or an issue with my lens. I guess I can mount up the lens on the T3i and see if I can repro the issue.

Otherwise - has anyone seen this before, and any idea as to whether it's a lens or camera body issue? If I need to send the body back, I have to do it tomorrow (I had a 7 day return window which ends Monday) so I need to make a decision about it today.

This is further compounded by me picking up a refurb 5D2 on sale yesterday, so I was planning to sell the 5D2 that I have now anyway, but I feel like if I suspect there is something wrong with it, it needs a trip to Canon. So perhaps that's another reason to return it to the seller and let them sort out what to do with it?

look for the AF-MF switch on the side of the lens... and try on the T3i as well!

Thanks!! Interesting idea that it could be the switch getting bumped. Def. could be that. Maybe when I was shooting handheld in portrait orientation, I was setting the switch half way by accident.

I walked the dogs and ran about 100 shots through the T3i and the 24-105L and could not repro the problem.

I checked my 5D2 and have taken approx. 155 images total with it since receiving it.

I've seen the AF-MF switch get stuck "in between" a few times on various lenses, giving an intermittent AF feeling... I suspect from rubbing when going in and out of bags, etc...

After playing with it for a bit, I also think it could be that I had the DOF button slightly depressed as it won't try to refocus if that's pushed in at all.

I don't usually shoot handheld, so who knows what weirdness I was up to.