steps to reproduce

• create a new class group "foo"
• create a new content class "bar"
• remember the <id-of-bar>
• create at least one content object of "bar"

• query the database: SELECT * FROM ezcontentobject WHERE contentclass_id=<id-of-bar>
• you should see the content objects of "bar"

• from "/class/grouplist" or "/class/classlist/<id-of-foo>" choose to remove the classgroup "foo"
• eZ warns that the content class "bar" and some objects will be removed as well
• do it

• query the database: SELECT * FROM ezcontentobject WHERE contentclass_id=<id-of-bar>
• now you can see the lurking undead

a simple fix is to use

eZContentClassOperations::remove($deleteClassID);

instead of

eZContentClass::remove();

in "kernel/class/removegroup.php"

as already suggested in "kernel/classes/ezcontentclass"