I don not use AV mode often on my 5d2 but it repeatedly has happened to me that I needed it in the middle of the action only to discover that it was set to underexpose by two stops and could not figure out any other way to fix this then to completely reset the whole camera which is of course a very undesirable thing to do, especially when in the middle of the action... Now today I finally understood why this happens to me, having auto-bracketing turned on, and as I do so often, forgetting about it, I switched to AV mode wanting to take a quick snap. Once ML starts bracketing it will set AV mode to underexpose... but how to reverse this??? (other then resetting the whole camera of course!)
After the bracketed sequence, the exposure parameters are reset to the old values (just checked).
Thanks a1ex, I guess I never found out cause I never let it finish...