For some reason I've started getting this notice when using field_group:
Notice: Uninitialized string offset: 0 in field_group_unpack() (line 1524 of /path/to/drupal/site/all/modules/contrib/field_group/field_group.module).
I have a feeling it was initially triggered when using commerce_fieldgroup_panes module in combination with features, but I haven't been able to track down the source of the issue. However, changing isset() to !empty() in field_group_unpack() seems to fix the notices for me. The attached patch makes that update.
PASSED: [[SimpleTest]]: [MySQL] 101 pass(es).
FAILED: [[SimpleTest]]: [MySQL] 74 pass(es), 27 fail(s), and 96 exception(s).
FAILED: [[SimpleTest]]: [MySQL] Unable to apply patch field_group-1596304.patch. Unable to apply patch. See the log in the details link for more information.