When multiple internet explorer tabbed windows that were opened, are closed by hitting 'X' on right corner of tabbed window. We see that the immediate window behind that increases its size, and there is a probability to close that window too by user without the need for it.
For example, open three tabbed windows in same IE browser window. Lets say you open Bing.com in first tabbed window that is right besides the browser URL bar. Next open Skype.com in same browser window. Skype will be second tabbed web page over Bing web page. Now open third tabbed web page. This will be over Skype webpage.
Now close the third opened webpage tabbed window. When you close it, you will see that Skype window that was opened second, size is increased and by mistake we may close that too though we don't want to. Try similar way with first opened webpage of bing.com by closing Skype.com webpage.