If you perform the steps to set the parent page of a page as itself, the following exception occurs:

java.lang.IllegalArgumentException: Can not set page as its own parent.
at com.atlassian.confluence.pages.Page.checkParentValid(Page.java:75)
at com.atlassian.confluence.pages.Page.setParentPage(Page.java:53)
at com.atlassian.confluence.pages.Page.addChild(Page.java:125)
at com.atlassian.confluence.pages.actions.EditPageAction.updateParentPage(EditPageAction.java:240)
at com.atlassian.confluence.pages.actions.EditPageAction.doSaveEditPageBean(EditPageAction.java:183)
at com.atlassian.confluence.pages.actions.EditPageAction.doEdit(EditPageAction.java:221)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
at java.lang.reflect.Method.invoke(Method.java:324)

Steps to reproduce (borrowed from CONF-4940):

1. start with an existing page
2. bring up the page update screen
3. enter the name of the page in the page's parent page field
4. save ('update')
5. EXPECT: validation error indicating that a page cannot be its own parent
6. reenter the name of the page in the parent page field, but this time with a different case
7. EXPECT: validation error indicating that a page cannot be its own parent