to reproduce:
0. install 6u10 b12, then uninstall 6u10 b12
1. download jre-6u5-windows-i586-p.exe or jdk-6u5-windows-i586-p.exe from http://jscpreview.sfbay/javase/downloads/index.jsp from either IE6x or IE 7
2. install jre-6u5-windows-i586-p.exe
3. after installation successfully, launch an applet, eg:
http://www.ibdprince.com/java.shtml
notice the applet wouldn't load, and no exception thrown on Java Consol

While with 6u10 (b12), the applet loaded successfully

Additioal problems:
when run an applet from the local hard drive,
C:\Program Files\Java\jdk1.6.0_05\demo\applets\Clock\example1.html
the applet won't work, and the message showed:
alt="Your browser understands the <APPLET> tag but isn't running the applet, for some reason." Your browser is completely ignoring the <APPLET> tag!

in addition, an information bar pop-up with:
"To help protect your security, Internet Explorer has restricted this webpage from running scripts or ActiveX controls that could access your computer. Click here for options..."

even we already turned off the related options.

This can be reproduced on at least two WinXP machines.

Note: Before installing 6u5 b13 bundles, the systems had 6u10 installed, and then un-installed.