I have tested it on Win XP. The same issue can exist on other OS as well.

How to reproduce:
1.) Install 6u10 RC build 27 of JRE
2.) Go to registry path [HKEY_LOCAL_MACHINE\SOFTWARE\JavaSoft\Java Update\Policy]
 and set this "UpdateServletURL"="http://sqindia.india.sun.com/disk17/deployment/pg152891/PIPTest/map.xml"
4.) Open JCP and go to update tab. Click on "Update Now" button. And Make sure JCP is still open.
5.) Proceed with update by clicking Install button. It will show File In use dialog, Proceed with the upgrade. Now you should have JRE 6u11 
6.) Uninstall Jre 6u11 from the Add/Remove.  Proceed with the uninstallation.
 Or Try to install 6u12 dummy bundle without uninstallaing anything
7.) Install 6u10 again.

Expected Behavior:
Install should go fine and JRE 6u10 or 6u12 should be installed

Actual Behavior:
Installation fails with Error "Error 25025: A previous Java uninstallation was never completed, you need to restart your computer before installing Java".

This could be because of  "PendingFileRenameOperations" under "HKEY_LOCAL_MACHINE\SYSTEM\ControlSet001\Control\Session Manager" has following registry key.

\??\C:\PROGRA~1\Java\jre6\bin\jusched.exe

\??\C:\PROGRA~1\Java\jre6\bin\jusched.exe

\??\C:\PROGRA~1\Java\jre6\bin\RTA00610
\??\C:\PROGRA~1\Java\jre6\bin\jusched.exe

This is working fine with b26. This seems to be regression from b26. I have checked in b26 and b24, there are no such issues.