When compiling using the useJavaUtilZip=true system option (that disables caching of the zip file indices in memory), the 6u25 javac crashes:
$ ./jdk1.6.0_25/bin/javac -J-DuseJavaUtilZip=true Test.java
error: error reading /tmp/jdk1.6.0_25/jre/lib/sunrsasign.jar; error in opening zip file
1 error

The javac version is:
$ ./jdk1.6.0_25/bin/javac -fullversion
javac full version "1.6.0_25-b05"

NetBeans is using this system option to prevent leaking memory through the zip file index cache: when a project is built inside NetBeans, the javac is run in the same VM to speed-up the building of small projects.