Another easy one. When you group anything multiple times and need to make a change and UNGROUP IT. All is lost and it ungroups everything.
If I have 3 groups of objects that are all grouped separately and need to group them all together but then I need to ungroup these. I am not left with the original groupings everything has been ungrouped.
Have it remember the pervious grouping so as you work you can ungroup and re group with out blowing up all the grouping.