Command prompt instance counting
Here's something that I accidently noticed today. Inside of a command prompt, of you type "cmd" (to launch a commonad prompt), it treats this as a new instance, though no new windows is opened. Now if you type "exit", the command window still stays since there were effectively two instances. Only when you type "exit" the second time, will the command window close.
This can go on at any level (I didn't really try too many levels). You can also easily make out the # instances by looking at the title bar. Every time you type "cmd" at a command prompt, you will see "- cmd" get added to the title. With every "exit" one of these goes away till you are back to the default title you had started up with and another "exit" will close the window.