#1 log in as admin in admin ( or admin2)
#2 Click "Setup"
#3 Click "Classes"
#4 Click "Content" group
#5 Edit "Article" class
#6 Save the article again (you don't need to do any changes to it)
#7 You'll now notice that you are redirected to wrong view (class view instead of class group view, probably because the class no longer has a group)

Verify that the class group is indeed wrong:
#1 Click "Classes" in left menu
#2 Click "Content" group
#3 Validate that Article is listed <--- this will fail