I am not sure removing the line will solve JamesC's problem. I tried Attaching and Detaching the CTXTabClientWnd in my app and it works fine as is. So the blow-up is probably for a different reason.

I also modified the GUI_OneNote sample to Detach and Attach the CTXTabClientWnd via a toolbar button and it works too. You have to make sure that CTXTabClientWnd is not attached when you try to attach it. Here is the code I used to toggle attach/detach

      if (m_MTIClientWnd.IsAttached())
      {
         m_MTIClientWnd.Detach();
      }
      else
      {
         m_MTIClientWnd.Attach(this);
      }

JamesC can you be more specific about where it blows up? Is it in MFC or XTreme Classes?

Remove the line

m_tab.m_pTabClientWnd=NULL;

in CXTPTabClientWnd::Detach().