Just a little Google fodder. Should you encounter this uncommon error message when trying to compile or run a Java program:
$ $ javac -version
Error occurred during initialization of VM
java/lang/ClassNotFoundException: error in opening JAR file /usr/local/java/jre/lib/rt.jar
it means that your installation of the JDK, and likely your original download is corrupt. Delete it, download again, and reinstall. This can occur with essentially any Java program: the interpreter, the compiler, jar, or a program merely written in Java such as LimeWire.
For reasons that aren’t especially clear, this bug seems to occur most frequently on Ubuntu Linux 7.10 Gutsy Gibbon when running in a virtualized environment such as VMWare. At least those seem to be the common factors for everyone else I found with this problem when googling it myself.