Using admin user on XP or Vista machine using a Standard user account,
1. Install 6u15 b03.
2. Make sure the jusched.exe process is running by starting it or logging out and logging in.
3. Patch-In-Place to 6u18 using http://j2se.east/arc/1.6.0_18/pit/2009-11-11/windows-i586/jre-6u18-windows-i586-iftw.exe
It looks like 6u15 to 6u18 PIP completed successfully.
BUT Actually:
A. Process Explorer shows jusched.exe is running.
B. C:\Program Files\Java\jre6\bin\jusched.exe's file version is 6.0.150.3 (6u15).
C. There is a pending file deletion of C:\PROGRA~1\Java\jre6\bin\jusched.exe under HKLM\SYSTEM\CurrentControlSet\Control\Session Manager, PendingFileRenameOperations.

4. Run http://j2se.east/arc/1.6.0_18/pit/2009-11-11/windows-i586/PLUS1/jre-6u19-windows-i586-iftw.exe to Patch-In-Place to 6u19.

Expected: I expected jre-6u19-windows-i586-iftw.exe to PIP to 6u19 without error.

Actual:
"Error 25025. A previous Java uninstallation was never completed. You need to restart your computer before installing Java."

The PIP from 6u15 to 6u18 left a PendingFileRenameOperation entry for C:\PROGRA~1\Java\jre6\bin\jusched.exe to be deleted at reboot.
Also happens on XP.
Make sure jusched.exe process is running before installing 6u18.