Open an xml file with a bad tag such as this one:
<entity.complex name='menu'>
<entity complex file='file_sgl.sgl.menu' />
</entity.complex>
The second line should have a red circle with an 'x' through it complaining about "Expected equal sign". Alt+Tab away from SE, fix the XML file in a different program, Alt+Tab back to SE. The file automatically updates (my SE updates all modified files automatically without prompting me) but the red 'x' is still there. I'm note sure how to make it go away, either.