Invalid page number (4) for the tab control named 'Control1'

I get this when I have a control, like a Frame or Subform on a Tab Control, then I end up changing my mind and deleting the Tab Page without first getting rid of the control. This is because the InPage property of the control no longer points to a valid page tab.

If the control is about the same size and the tab control, you may miss the fact that it is still on the page.