Please download the attached example and unpack it. Then go to directory "flowtest" and run:
javac -cp FlowTest2.jar flowtest3/Main.java

The following exception is reported for me:
An exception has occurred in the compiler (1.6.0-rc). Please file a bug at the Java Developer Connection (http://java.sun.com/webapps/bugreport) after checking the Bug Parade for duplicates. Include your program and the following diagnostic in your report. Thank you.
java.lang.NullPointerException
        at com.sun.tools.javac.comp.Flow.visitApply(Flow.java:1117)
        at com.sun.tools.javac.tree.JCTree$JCMethodInvocation.accept(JCTree.java:1209)
        at com.sun.tools.javac.tree.TreeScanner.scan(TreeScanner.java:35)
        at com.sun.tools.javac.tree.TreeScanner.visitExec(TreeScanner.java:157)
        at com.sun.tools.javac.tree.JCTree$JCExpressionStatement.accept(JCTree.java:1073)
        at com.sun.tools.javac.tree.TreeScanner.scan(TreeScanner.java:35)
        at com.sun.tools.javac.comp.Flow.scanStat(Flow.java:484)
        at com.sun.tools.javac.comp.Flow.scanStats(Flow.java:492)
        at com.sun.tools.javac.comp.Flow.visitBlock(Flow.java:747)
        at com.sun.tools.javac.tree.JCTree$JCBlock.accept(JCTree.java:738)
        at com.sun.tools.javac.tree.TreeScanner.scan(TreeScanner.java:35)
        at com.sun.tools.javac.comp.Flow.scanStat(Flow.java:484)
        at com.sun.tools.javac.comp.Flow.visitTry(Flow.java:948)
        at com.sun.tools.javac.tree.JCTree$JCTry.accept(JCTree.java:966)
        at com.sun.tools.javac.tree.TreeScanner.scan(TreeScanner.java:35)
        at com.sun.tools.javac.comp.Flow.scanStat(Flow.java:484)
        at com.sun.tools.javac.comp.Flow.scanStats(Flow.java:492)
        at com.sun.tools.javac.comp.Flow.visitBlock(Flow.java:747)
        at com.sun.tools.javac.tree.JCTree$JCBlock.accept(JCTree.java:738)
        at com.sun.tools.javac.tree.TreeScanner.scan(TreeScanner.java:35)
        at com.sun.tools.javac.comp.Flow.scanStat(Flow.java:484)
        at com.sun.tools.javac.comp.Flow.visitMethodDef(Flow.java:693)
        at com.sun.tools.javac.tree.JCTree$JCMethodDecl.accept(JCTree.java:638)
        at com.sun.tools.javac.tree.TreeScanner.scan(TreeScanner.java:35)
        at com.sun.tools.javac.comp.Flow.visitClassDef(Flow.java:627)
        at com.sun.tools.javac.tree.JCTree$JCClassDecl.accept(JCTree.java:574)
        at com.sun.tools.javac.tree.TreeScanner.scan(TreeScanner.java:35)
        at com.sun.tools.javac.comp.Flow.analyzeTree(Flow.java:1255)
        at com.sun.tools.javac.main.JavaCompiler.flow(JavaCompiler.java:1014)
        at com.sun.tools.javac.main.JavaCompiler.flow(JavaCompiler.java:988)
        at com.sun.tools.javac.main.JavaCompiler.compile2(JavaCompiler.java:697)
        at com.sun.tools.javac.main.JavaCompiler.compile(JavaCompiler.java:662)
        at com.sun.tools.javac.main.Main.compile(Main.java:327)
        at com.sun.tools.javac.main.Main.compile(Main.java:253)
        at com.sun.tools.javac.main.Main.compile(Main.java:244)
        at com.sun.tools.javac.Main.compile(Main.java:69)
        at com.sun.tools.javac.Main.main(Main.java:54)

Occurs on:
$ java -version
java version "1.6.0-rc"
Java(TM) SE Runtime Environment (build 1.6.0-rc-b87)
Java HotSpot(TM) Client VM (build 1.6.0-rc-b87, mixed mode, sharing)

The exceptions is not thrown on:
$ java -version
java version "1.5.0_06"
Java(TM) 2 Runtime Environment, Standard Edition (build 1.5.0_06-b05)
Java HotSpot(TM) Client VM (build 1.5.0_06-b05, mixed mode, sharing)

the following compilation error is reported on the java version "1.5.0_06", which is correct, as package flowtest1 and its content is not on the classpath:
flowtest3/Main.java:29: package flowtest1.Main does not exist
            flowtest1.Main.X x = null;
                          ^
1 error