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I'm running the 1.x-dev branch and running into issues with nested field groups where they are out of order by default; ie: "child, child, parent, child".
Presumably this order is based on the element's creation order because it doesn't take the weight into account.
In field_group_fields_nest(), even though the hierarchy gets resolved properly and stored in $group_references, the groups are still passed to field_group_pre_render() in their original order , so in our case, the parent will get rendered before the last child. This obviously means that the display gets broken.