Pressing ALT+SHIFT changes the language of the keyboard on windows systems (for example to write US-English and Greek). But the ALT key triggers also the menu. Now if you try to change the language while typing into a text form field or the location bar, the language indeed changes but it isn't possible to continue typing, as the menu has been activated. You have to click again with the mouse to the position you were writing.
dupe of bug 39721
*** This bug has been marked as a duplicate of 39721 ***
I'm seeing this also. Windows 2000. BuildID: 2001062815. This one is certainly a showstopper for people using foreign languages.