Closing Windows and Confirmation Boxes
Ever notice how a script that attempts to close a window will at times generate a confirmation and other times not?

Here's why: IE for windows and NN 4.x browsers do not allow a non-JavaScript-generated window with more than one page in its history to be closed without asking the user for permission via a confirmation box. In other words, scripts cannot close a main window unconditionally.

The MSIE Work Around
To determine whether or not a window is a main window, Internet Explorer checks to see if the window has an opener. Since windows that have openers can be closed unconditionally, the solution is to make Explorer think a main window has an opener. That is accomplished by giving a phantom opener a value, in this case, an empty string:

Something I didn't even know to go looking for before I read the IE7 release notes.