A testcase is provided by the customer to reproduce the problem.
Customer is seeing the java plugin fatal error on
both IE and Firefox, and in following versions of
jdk 1.4.2_13, jdk 5.0, and jdk 6.0 on windows platform.

IMPT NOTE: This works with MS JVM but not with Sun's JVM.

The error is "Cannot load class sun/plugin/JavaRunTime"
in the error dialog box.
Searching the web reveals a few bugs that are likely
candidate:
6184339 Synopsis Java Plugin Fatal Error- Can Not load class
sun/plugin/JavaRunTime

6175601

5017973 Unsupported OBJECT tag for JRE 1.3.1 and JRE 1.4.2

4523129

However, the workarounds mentioned does not resolve the issue.