1. Create two objects (e.g. rectangles)
2. Group them
3. Open Appearance panel
4. Add a stroke to your group
5. Remove the stroke (with the Delete Selected Item button)
6. Open Layers panel and study the appearance 'ring' marker of the group — Illustrator now believes the group has a custom appearance, while there is none!

If you click the Contents in the Appearance panel with the group selected, Illustrator will clearly say 'Group: No Appearance' in the top of the panel.

This 'empty' appearance can be saved as Graphic Style even and applied to another object, giving no actual result, except for the 'solid ring' state again.

This behavior leads to a problem with exporting the group as a vector shapes to PSD format.