When I invoked the attached test case and tried the following reproducing way,
ALT+TAB is recongnized as mouse-click.

CONFIGURATION :
OS : WindowsXP prof. (SP3, Japanese)
Browser : IE6(SP3) and IE7(SP3)
JDK : 5.0u23 and later

REPRODUCE :
1. Invoke the attached test_applet.html and test_applet.class in IE
2. Open some other windows, ex. text box, explore,...
   (Please see the attached sample-image.jpg just as a sample.)
3. Keep clicking the "test" button in the browser and change the focus to another windows
   (make another window active) with ALT+TAB

The selected another window becomes "active", the count of label(represents num. of clicking button) is updated.
This should not be updated.

NOTE:
-This problem occurs in 5u23(u24 and u25), not in jdk5u22.
-This does not occur in JDK6u23.