If you have a create a group with at least one layer, then change the Fill of the group to anything other than 100%, that value isn't saved for the next time that you open that file. It remembers the opacity, paint type (pass through is default), and any style effects put on the layer, but it doesn't remember the Fill setting.
I found this in the beta and an adobe employee said it would be fixed as soon as possible... it's still not fixed. It's not a critical bug, but I wanted it documented that it still exists. Here's the original thread (now read only) http://forums.adobe.com/thread/984900