When you are running javadoc on a class which fails to provide
an implementation for one of the methods that is specified by an
interface which it claims to implement, then javadoc will get an
exception. For example:

----begin testdoc.java----
class testdoc implements java.awt.image.ImageObserver {
}
----end testdoc.java----

Note that the ImageObserver interface requires a function that
takes some parameters. If you change ImageObserver above to
"Runnable" there will be no error since the method required by
Runnable takes no parameters.

Run javadoc on the testdoc.java file and it should get the error:

javadoc -d . -verbose testdoc.java
[parsed testdoc.java in 130ms]
generating documentation for class testdoc
[loaded /export/home/benden0/java/build/classes/java/lang/Object.class in 86ms]
[loaded /export/home/benden0/java/build/classes/java/awt/image/ImageObserver.class in 38ms]
java.util.NoSuchElementException VectorEnumerator
at java.lang.Throwable.<init>(Throwable.java:70)
at java.lang.Exception.<init>(Exception.java)
at java.lang.RuntimeException.<init>(RuntimeException.java)
at java.util.NoSuchElementException.<init>(NoSuchElementException.java)
at java.util.VectorEnumerator.nextElement(Vector.java)
at java.tools.javadoc.DocumentationGenerator.genMethodDocumentation(DocumentationGenerator.java:495)
at java.tools.javadoc.DocumentationGenerator.genClassDocumentation(DocumentationGenerator.java:990)
at java.tools.javadoc.Main.main(Main.java:216)
error: An exception has occurred in the compiler, please file a bug report (###@###.###).