The following results in a uncaught error from apt:

mkdir foo
touch foo/bar.class
apt -factory foo/bar

An exception has occurred in apt (1.5.0_01). 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.NoClassDefFoundError: IllegalName: foo/bar
        at java.lang.ClassLoader.preDefineClass(ClassLoader.java:476)
        at java.lang.ClassLoader.defineClass(ClassLoader.java:614)
        at java.security.SecureClassLoader.defineClass(SecureClassLoader.java:124)
        at java.net.URLClassLoader.defineClass(URLClassLoader.java:260)
        at java.net.URLClassLoader.access$100(URLClassLoader.java:56)
        at java.net.URLClassLoader$1.run(URLClassLoader.java:195)
        at java.security.AccessController.doPrivileged(Native Method)
        at java.net.URLClassLoader.findClass(URLClassLoader.java:188)
        at java.lang.ClassLoader.loadClass(ClassLoader.java:306)
        at java.lang.ClassLoader.loadClass(ClassLoader.java:251)
        at com.sun.tools.apt.comp.Apt.main(Apt.java:287)
        at com.sun.tools.apt.main.JavaCompiler.compile(JavaCompiler.java:458)
        at com.sun.tools.apt.main.Main.compile(Main.java:1075)
        at com.sun.tools.apt.main.Main.compile(Main.java:938)
        at com.sun.tools.apt.Main.processing(Main.java:95)
        at com.sun.tools.apt.Main.process(Main.java:43)
        at com.sun.tools.apt.Main.main(Main.java:34)

The cause is the catch blocks for

AnnotationProcessorFactory factory =
(AnnotationProcessorFactory) (aptCL.loadClass(factoryName).newInstance());
list.add(factory);

do not check for the NoClassDefFoundError that can be thrown by the classloader.